London Underground Signalling - Tubeprune Course Sig v2.pdf · London Underground Signalling 1 - Systems Before ATO “A Crash Course in London Underground ......

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London Underground Signalling 1 - Systems Before ATO

“A Crash Course in London Underground Signalling”

by Piers Connor

Part 1, Draft 2

Date: 15th July 2010

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1 - Brakes and Blocks Introduction Railway  control  and  signalling  systems  and  the  associated  communications  systems  (CoCoSig  as  it’s  called  now),  play  a  vital  part  in  the  safe  operation  of  railways.    Nowhere  is  this  more  important  than  on  intensely  trafficked  suburban  and  urban  railways  like  the  London  Underground,  where  vital  safety  systems  of  train  control  allow  the  railway  to  move  over  3  million  passengers  a  day  efficiently  and  safely.    This  booklet  examines  the  background,  development,  engineering  and  operation  of  signalling  on  the  London  Underground  and  shows  how  it  developed  from  a  pioneer  safety  system  at  the  start  of  the  20th  Century  to  pioneering  full  automatic  train  operation  in  the  1960s  and  from  there  into  the  modern  train  control  systems  of  the  21st  Century  that  are  now  being  installed.      

It  may  be  that  some  readers  have  no  previous  long  term  or  detailed  experience  in  railway  signalling  so  here  we  cover  some  basics  which  will  be  well  known  to  others.    In  addition,  it  should  be  understood  that  there  are  subtleties  in  the  basic  approaches  to  the  LU  system  that  differ  from  main  line  practice,  so  references  to  main  line  signalling  are  included  for  comparison  where  relevant.  

Brakes Any  proper  understanding  of  signalling  has  to  begin  with  an  understanding  of  the  limitations  of  train  braking.    Signalling  was  introduced  as  a  means  of  preventing  train  collisions.    It  was  realised  very  early  on  in  the  development  of  

railway  operation  that  trains  were  not  easy  to  stop  quickly.    There  were  two  reasons  for  this.    Firstly,  train  brakes  were  not  continuous,  that  is,  not  all  vehicles  were  braked  and  those  that  were  only  had  manually  operated  brakes.    Braking  was  a  haphazard  business  at  best,  relying  on  whistle  signals  from  the  driver  to  call  for  brakes  and  then  on  the  brakesmen  to  actually  provide  them.    It  didn’t  always  work  very  well,  especially  as  trains  got  heavier  and  speeds  rose.    Things  were  eventually  resolved  in  the  late  19th  century  by  the  adoption  of  continuous  brakes,  which  were  more  technically  efficient,  which  were  remotely  controlled  by  the  driver  and  which  could  be  used  in  emergency  by  other  members  of  staff.      

The  second  problem  for  train  braking  was  the  adhesion  factor  between  wheel  and  rail.    This  was  much  less  easy  to  resolve.      

Adhesion As  any  train  driver  will  tell  you,  driving  a  train  is  easy.    The  difficult  bit  is  stopping  it.    It’s  normally  easy  to  get  a  train  going  but  it’s  much  more  difficult  to  stop  it,  particularly  to  stop  it  in  the  right  place.    To  do  this  consistently,  whilst  avoiding  damage  to  the  train,  giving  a  reasonably  comfortable  ride  to  the  passengers  and  keeping  time,  requires  skill  and  concentration.    The  reason  for  this  is  simple  –  the  adhesion  available  for  a  train  with  a  steel  wheel  on  a  steel  rail  is  such  that  the  braking  distance  is  considerably  more  than  you  get  in  a  car  with  rubber  tyres  on  the  average  road.    The  adhesion  between  a  tyre  and  the  road  surface  can  be  measured  at  over  80%.    The  main  line  railways  calculate  their  braking  distances  on  the  basis  of  8%  adhesion,  an  order  

of  magnitude  less.    So,  if  you  are  driving  your  car  at  70mph  and  you  need  to  stop,  you  think  about  it  (say  20m)  and  then  brake  (75m),  total  90m.    Now  transfer  yourself  to  the  cab  of  a  train  at  70  

Fig. 1: Schematic illustrating the calculation of the braking distance required by a main line train travelling at 125 mph (200km/h or 55m/s). The formula is v2/2b where v is the initial speed of the train and b is the average deceleration rate. The brake rate is set at a comfortable 0.7m/s2. A sighting distance, calculated from the usual time allowed of 10 seconds, must be added to the total. Note that the signal can be referred to as the limit of movement authority or LMA. Diagram after F Schmid.

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mph.    Think  about  it  (20m),  apply  the  brake  and  wait  for  it  to  feed  up  to  the  required  pressure  on  the  whole  train  (60m),  then  wait  for  the  train  to  stop  (950m  approx.),  a  total  1030m,  or  more  than  a  kilometre.    An  emergency  brake  application  at  this  speed  might  get  the  train  to  a  stand  in  700m,  if  it’s  not  raining.    This  means  that  a  train’s  braking  distance  is  more  than  ten  times  greater  than  that  of  a  car  and  (trust  me)  it  seems  like  a  million  times  greater  when  you  are  at  the  front  of  a  train  with  the  emergency  brake  fully  applied  and  you  know  there  is  nothing  you  can  do  to  avoid  hitting  something  in  front  of  you.  

All  railways  –  with  the  exception  of  those  few  using  rubber  tyres,  which  have  rather  different  problems  –  suffer  from  this  problem  of  adhesion.    Although  we  could  reasonably  expect  to  get  at  least  20%  adhesion  in  the  dry  tunnels  of  the  London  Underground,  in  the  open,  where  there  is  always  the  likelihood  of  moisture,  grease,  ice  and  even  leaves,  the  current  adhesion  limit  assumed  is  6.5%  for  the  design  of  automatic  systems.    What  all  this  tells  us  is  that,  since  trains  can’t  stop  instantly  or  even  very  quickly,  you  need  a  lot  of  advance  warning  of  when  you  are  closing  in  on  a  train  ahead.    Satisfactory  ways  of  doing  this  took  a  long  time  to  develop.  

Blocks Early  systems  used  to  pass  trains  from  station  to  station,  the  line  between  two  stations  effectively  becoming  a  section1  or,  as  it  is  described  on  main  line  railways,  a  “block”.    A  train  was  only  allowed  to  leave  the  station  at  the  entrance  of  the  block  when  it  was  confirmed  that  the  train  ahead  was  

1 On a standard two-track railway, it is two sections, one for each direction.

clear  of  it.    A  visual  signal  was  placed  at  the  entrance  to  the  block  to  show  the  driver  whether  it  was  OK  to  proceed.    Soon,  a  rule  evolved,  which  is  still  valid  today  -­‐  “only  one  train  is  allowed  in  any  one  section  at  any  one  time”.    Obviously  there  are  carefully  managed  waivers  for  coupling,  rescue  and  the  like  but  that’s  the  basic  rule.  

Gradually,  this  arrangement  was  developed  so  that  the  signalman  controlled  the  area  around  his  station  and  the  admission  of  trains  into  the  sections  approaching  his  station  (Fig.  2).    The  whole  operation  was  manual  and  relied  on  the  vigilance  of  the  staff  to  make  it  work  safely.    The  signalman  (nowadays  called  a  “signaller”,  in  case  he’s  a  she,  or  on  LU,  “Service  Controller”)  had  to  watch  out  until  he  saw  the  train  depart  his  area  of  control  before  he  would  pass  the  “train  out  of  section”  message  to  the  signalman  in  rear  and  subsequently  “accept”  another  train  when  offered  it.      

Fig. 2: Diagram of the traditional layout of block signaling showing the area of control managed by a signal box. The Starting signal at each station protected the entrance to the block section. This signal could not be cleared until permission was granted by the signal box at the next station. The Home signal protected the station approach. The Distant signal provided advance warning of the home signal. The area between the home and starting signals was “station limits and was under the local control of the signal box.

On and Off

Amongst railway people, signals are said to be “on” or “off”. “On” means the signal is showing a stop indication while “off” means it is showing a proceed indication. These descriptions refer back to the very early days of railways when a signal often consisted of a board on a post. If you wanted to tell the driver to wait, you placed the board on the post, facing the driver, so the signal was “on” the post. When it was OK for the driver to go, you took the board “off”.

Even today, some main line drivers still refer to signals as “boards”. In some places, like the Midlands, they are called “pegs”. On the Underground, they are referred to as “sticks”. I have no idea why.

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2 - Detection and Protection Train Detection

During  many  years  of  development,  the  signalman  was  gradually  provided  with  electrical  and  mechanical  interlocks  and  “instruments”  of  various  types,  which  were  designed  to  help  prevent  him  from  making  some  basic  errors  and  to  remind  him  of  the  status  of  the  blocks  he  controlled.    Train  detection  was  visual,  signalmen  using  what  aircraft  pilots  refer  to  as  “the  Mark  1  eyeball”  to  confirm  the  passing  of  trains.    This  worked  OK  as  long  as  everyone  was  paying  attention  and  things  didn’t  get  too  busy  but  a  number  of  accidents  over  the  years  showed  that  it  wasn’t  foolproof.    Signalmen  could  and  did,  occasionally  let  trains  run  into  occupied  sections  with  sometimes  fatal  results.    It  was  from  these  incidents  that  the  idea  of  some  sort  of  automatic  train  detection  was  thought  desirable  and,  by  the  late  19th  century,  it  had  arrived  in  the  form  of  the  track  circuit,  the  principle  of  which  is  shown  in  Fig.  3  above.    

The Track Circuit A  track  circuit  is  just  a  low  voltage  circuit  fed  into  a  section  of  track.    A  train  is  detected  when  it  

occupies  the  track  and  shorts  out  the  circuit.    The  system  could  be  used  to  prevent  errors  by  signalmen  operating  signal  levers  or  it  can  be  used  to  operate  the  signal  itself.    Originally  it  was  battery  operated,  using  direct  current  but  it  later  evolved  into  a  more  sophisticated  system  using  an  AC  supply.      

If  you  pass  a  low  voltage  (5-­‐10v  AC)  electric  circuit  through  the  running  rails  you  can  use  the  circuit  to  operate  a  relay  to  indicate  current  is  flowing.    If  current  is  flowing  the  relay  can  be  used  to  switch  on  a  light  -­‐  in  our  case  a  green  signal  lamp  lit  by  100v  AC.    If  the  current  stops  flowing,  the  relay  contacts  drop  to  switch  the  feed  to  the  red  light.    The  circuit  is  normally  arranged  so  that  the  feed  is  at  the  far  end  and  the  relay  at  the  entry  end  of  the  circuit.    This  gives  continuity  to  the  detection  ahead  of  the  train  while  it  passes  through  the  section.  

Since  the  wheels  and  axles  of  train  are  steel,  they  short  circuit  the  track  as  soon  as  they  enter  the  section  and  you  can  use  the  loss  of  the  circuit  to  show  the  section  is  occupied.    The  track  circuit  “goes  down”  as  they  say,  the  relay  drops  open  and  the  signal  shows  a  red  light  to  the  driver  of  

Fig. 3: Schematic of simple track circuit showing the arrangement for automatically operating the signal protecting the section. The diagram shows the section occupied by a train and the signal at danger. If there was no train in the section, the circuits would operate as shown by the dotted lines and the signal would show a green aspect. In reality, many sections have an insulated block joint in one rail only but the principle is the same. After Japan Railway and Transport Review.

Fig. 4: Diagram showing the basic arrangement of block sections on a London Underground plain track. Each signal stands at the entrance to the block it protects. The signals operate automatically and are usually identified by the letter A followed by a number. Signals are numbered, where possible, with even numbers in one direction and odd numbers in the other.

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the  next  train  to  approach.    When  the  train  leaves  the  section,  the  circuit  “picks  up”  and  the  relay  is  energised  and  switches  the  signal  to  green.    If  you  divide  the  line  up  into  sections  and  isolate  each  section  electrically  from  its  neighbours  by  means  of  “insulated  block  joints”,  the  whole  line  can  be  protected  with  this  system  of  “automatic  signalling”  (Fig.  4  above).      

A  word  of  caution  here:    A  track  circuit  doesn’t  actually  detect  the  presence  of  a  train,  it  detects  the  absence  of  a  train.    As  long  as  the  circuit  is  complete,  you  can  be  pretty  sure  there  isn’t  a  train  there.    If  the  circuit  isn’t  complete,  there  might  be  a  train  there.  

The  track  circuit  was  adopted  by  the  Underground  in  1903  and,  eventually,  by  many  other  railways  and  it  is  still  the  principal  form  of  train  detection  today.    Modern  versions  have  become  available,  for  example  where  adjacent  circuits  use  different,  electronically  generated  frequencies  and  avoid  the  need  for  insulated  joints  –  jointless  track  circuits  or  JTCs.    Then  there  are  axle  counters,  which  don’t  require  track  circuits  but  which  adopt  different  rules.    More  on  these  later.  

The Trainstop An  additional  feature  of  the  Underground’s  signalling  system,  installed  at  the  same  time  as  automatic  signals,  is  the  trainstop.    Each  signal  where  a  train  might  be  required  to  stop,  is  provided  with  a  mechanical  device  adjacent  to  the  right  hand  running  rail.    The  device  consists  of  an  arm,  which  operates  in  conjunction  with  the  signal,  being  raised  to  stop  a  train  which  attempts  to  pass  if  the  signal  shows  a  stop  aspect.    The  train  is  provided  with  a  “tripcock”,  matching  the  location  of  the  trainstop,  which  is  connected  to  the  braking  system.    If  the  tripcock  is  operated  by  the  trainstop,  it  causes  an  “irretrievable”  emergency  brake  application,  i.e.  the  tripcock  cannot  be  reset  until  the  train  has  stopped.      

The  trainstop  is  raised  by  a  spring  and  lowered  by  a  compressed  air  cylinder  arrangement,  using  air  supplied  from  a  trackside  pipe  –  the  air  main  -­‐  see  box  on  next  page.    This  design  ensures  that  the  loss  of  air  supply  will  cause  the  trainstop  arm  to  be  raised.      

The  use  of  automatic  signals  with  trainstops  for  over  100  years  has  made  the  Underground  one  of  the  safest  railways  in  the  world.    Nevertheless,  

the  use  of  a  complex  mechanical  device,  coupled  to  the  automatic  signalling  system,  does  not  come  without  a  price.    The  air  supply,  trainstops,  interfaces  and  associated  train  equipment  all  require  regular  checking  and  maintenance  and  this  presents  a  considerable  manpower  burden.    Modern  systems  of  train  control  can  eliminate  the  system.  

Overlaps Now  that  each  section  is  protected  by  a  stop  signal  and  a  trainstop,  you  might  be  forgiven  for  thinking  that  trains  were  now  safe  from  being  run  into  from  behind.    Unfortunately,  this  is  not  so.    As  we’ve  seen  above,  a  train  needs  lots  of  room  to  brake  and,  if  it  passes  a  signal  showing  a  stop  command  and  gets  tripped,  there  will  be  quite  a  distance  before  it  comes  to  a  stand.    If  the  train  in  

Fig. 6: Close-up of the trainstop/tripcock interface used on London Underground. The trainstop arm is shown raised behind its equipment box. The tripcock lever mounted on the train is shown in white. The trainstop arm is raised by spring operation and lowered by compressed air supplied from the trackside.

Fig. 5: View of London Underground automatic, 2-aspect signal, with associated trainstop mounted on the right hand side of the right hand running rail. When the signal shows a stop aspect, the trainstop arm is raised and when a proceed aspect is shown the trainstop arm is lowered. The air supply to the trainstop is carried by a pipe running alongside the track and is passed to the trainstop as required through an electrically operated valve.

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the  section  protected  by  that  signal  is  just  inside  the  section,  a  short  distance  beyond  the  signal,  the  tripped  train  will  hit  it  before  it  stops  (Fig.  7  below).      

To  remove  this  possibility,  you  have  to  give  the  tripped  train  room  to  stop  before  it  enters  the  section.    This  means  it  must  be  tripped  before  it  reaches  the  section;  in  fact,  it  must  be  tripped  a  full  braking  distance  before  it  reaches  the  section.    So,  to  provide  this  distance,  you  have  to  move  the  signal  and  its  trainstop  a  safe  braking  distance  back  from  the  entrance  to  the  section.    The  distance  the  signal  has  to  be  moved  back  is  called  the  overlap  (Fig.  8  below).  

On  London  Underground,  overlaps  are  calculated  on  a  site  by  site  basis.    Over  the  years,  a  complex  

formula  has  evolved  which  takes  into  account  the  gradient,  the  maximum  possible  train  speed,  the  braking  capacity,  several  margins  for  error  and  even  the  position  of  the  tripcock  relative  to  the  front  of  the  train.    In  broad  terms  it  works  out  as  the  emergency  braking  distance  plus  about  30%.  

In  many  locations,  the  calculated  overlap  is  longer  than  a  train’s  length,  so  it  would  be  possible  for  the  train  to  pass  the  signal  entirely  before  entering  the  section  that  the  signal  is  protecting.    Until  the  train  enters  the  section  and  the  track  circuit  “goes  down”,  the  signal  shows  a  green  aspect  behind  the  train  –  not  good  practice.    To  eliminate  this  possibility,  the  overlap  has  its  own  track  circuit  (the  “replacement  track”)  so  that  the  signal  will  return  to  danger  as  soon  as  the  leading  wheelset  of  the  train  passes  it.    This  feature  means  that  most  Underground  signal  sections  have  two  track  circuits  and  this  is  

reflected  in  the  LU  track  circuit  numbering  system  for  automatically  signalled  areas  so  that,  in  our  example  of  Section  123  (Fig.  7),  the  track  circuits  would  be  numbered  123a  and  123b,  the  latter  being  the  overlap  of  Signal  A125.    Both  track  circuits  still  form  the  section  123  and  both  must  be  clear  for  Signal  A123  to  show  a  green  aspect.  

Fig. 7: Schematic demonstration of how a tripped train (Train 2) could enter an occupied section and collide with a train ahead (Train 1). This is prevented by what is called an “overlap”, Fig. 8 below.

Fig. 8: Schematic showing how each signal is positioned a safe braking distance back from the entrance to the section it protects to allow room for a tripped train to stop. This distance is called the overlap. Note that the overlap normally has its own track circuit to ensure that the signal returns to danger as soon as the front of the train passes it. Each block section is here shown in colour with the LU track circuit numbering convention.

Air Main

The problem with air operation of trainstops and points is that you need a compressed air supply, which will be available all over the system, so an “air main” has to provided along each side of each route. London Underground has the world’s largest interconnected compressed air system. Compressors in the traction substations supply the air main at 60 psi (about 4 bar). Naturally, the air supply system is expensive to maintain, sensitive to extreme weather conditions and prone to failure. Around the system, there are 48 compressors working just to keep the pressure up against leaks in the pipework.

LU wants to get rid of the air main but it will only be able to do so when all trainstops and air operated point machines have gone. On the Central Line, which now has ATO and electro-hydraulic points, much of the air main has been removed but it is still required in some places for the old “P” type fare gates – the ones that open quickly with a loud “clunk”.

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Repeaters As  trains  need  a  lot  of  distance  to  stop,  drivers  need  to  know  well  in  advance  about  the  condition  of  the  signals  ahead  of  them.    In  many  cases  they  can’t  see  the  signal  early  enough  to  be  able  to  stop  at  it  if  it  is  showing  a  danger  aspect.    This  problem  was  recognised  very  early  on  in  the  development  of  railways,  so  drivers  were  provided  with  “Distant”  signals.    The  Distant  repeated  the  indication  of  the  stop  signal  and  gave  the  driver  a  good  chance  of  stopping  if  he  needed  to.      

The  Underground  adopted  the  same  idea,  but  modified  to  suit  its  close  headways  and  tunnel  conditions.    On  main  line  railways  using  semaphore  signals,  distants  are  normally  used  in  specific  circumstances  regardless  of  sighting  but,  on  the  Underground,  repeaters  are  only  provided  where  sighting  of  the  stop  signal  is  compromised  in  some  way.    Since  the  standard  Underground  stop  signal  is  a  two-­‐aspect,  red  or  green  signal,  repeaters  for  automatic  signals  are  two-­‐aspect  signals  showing  green  or  yellow  aspects,  depending  on  the  aspect  of  the  stop  signal. Repeaters  for  automatic  signals  are  lettered,  on  a  yellow  plate,  “R”  plus  the  number  of  the  associated  stop  signal.  

Although  a  repeater  is  designed  to  repeat  the  aspects  of  its  associated  stop  signal,  there  is  a  period  when  it  doesn’t.    If  a  signal  and  its  repeater  are  both  showing  green2  and  a  train  approaches,  it  will  pass  the  repeater  first.    As  it  carries  on,  it  is  likely  to  be  fully  past  the  repeater  before  it  reaches  the  stop  signal.    This  means  the  repeater  is  showing  green  behind  the  train.    To  

2 A repeater will show green when the signal it repeats shows green and its trainstop is lowered.

prevent  this,  like  the  overlap,  the  repeater  has  its  own  “replacement”  track  circuit  so  that,  as  the  train  passes  it,  it  changes  from  green  to  yellow.    As  a  result,  in  the  short  time  the  train  is  between  the  repeater  and  the  signal  it  repeats,  the  signal  will  show  green  while  its  repeater  shows  yellow,  as  shown  in  Fig.  9.    So,  you  could  say  that  a  repeater  repeats  the  aspects  of  its  associated  stop  signal  but,  er,  not  all  the  time.  

“Well”,  you  might  ask,  “Why  go  to  this  trouble  to  prevent  a  green  light  showing  behind  a  train?    After  all,  there  is  a  red  signal  protecting  the  section  it  occupies  (A123  in  Fig.  9),  so  there  won’t  be  any  way  the  driver  of  a  following  train  can  see  the  green  of  the  repeater.”    Well,  actually,  there  is.    It’s  called  the  “Stop  and  Proceed  Rule”.    

Fig. 9: Schematic of section of line showing how a repeater R125 shows a yellow aspect while its associated stop signal shows green using an additional track circuit. This ensures that a train will not have a green signal showing immediately behind it. Section 123 (shown in orange) now has three track circuits.

The First Automatic Signals

Automation of signalling, using track circuits, came to the Underground early. An automatic signal, using a track circuit operated by the passage of trains instead of by a signalman using a lever connected to it by a long wire, was tried by the District Railway on the Hounslow branch in 1901. It was imported from the US (as was much of the original Underground technology) and it was successful enough for a complete system of them to be tried on the Ealing & South Harrow Railway (now Rayners Lane branch as far as South Harrow) when it was electrified in 1903. This too was successful and the Underground adopted it for its new tube lines and for the electrification of the District Railway. The Metropolitan also adopted automatic signalling eventually but it was never fully integrated with the other lines until it was taken over by London Transport in 1933.

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3 – Rules and Enforcement Stop & Proceed It  was  discovered  very  early  on  that  automatic  signals  don’t  always  behave  themselves.    Now  and  then,  they  would  fail  to  clear  after  the  train  had  left  the  section  they  were  protecting.    The  “signal  failure”,  bane  of  the  commuter,  had  arrived.    Actually,  a  signal  staying  at  red  when  the  train  isn’t  in  the  section  is  not  actually  a  failure.    It  is  the  system  degrading  to  the  next  level  of  safety.    Something  hasn’t  done  what  it  was  it  is  supposed  to  do  so  the  signal  isn’t  about  to  let  trains  go  past  it  to  enter  a  section  which  might  be  occupied.    It  stays  at  red.    It’s  inconvenient  but  safe.  

In  the  early  days  of  Underground  electric  train  operation,  there  was  no  radio  and  few  phones.    Trains  in  tunnels  were  effectively  isolated  from  the  rest  of  the  world.    To  allow  some  sort  of  contact  to  be  made  in  the  event  of  a  signal  remaining  at  danger,  the  stop  and  proceed  rule  was  introduced.    This  rule  told  the  driver  to  wait  one  minute  (nowadays  two)  and  then  proceed  past  the  signal  slowly,  basically  driving  on  sight.    The  phrase  “extreme  caution”  was  developed.    Of  course,  the  train  would  be  tripped.    The  driver  had  to  reset  the  tripcock  and  then  move  off  very  slowly  into  what  might  be  an  occupied  section.  

This  process  achieved  two  objectives.    It  allowed  a  train  stuck  at  a  red  signal  to  move  forward  to  the  next  station  at  least  and  let  everyone  know  at  “Euston,  we  have  a  problem”.    Also,  it  provided  a  means  for  a  stalled  train  to  be  approached  and  perhaps,  be  assisted  by  the  next  train  –  the  classic  “push  out”.    Both  have  been  used,  the  former  the  most  regularly.    Today,  with  train  radio,  the  process  is  a  bit  easier.    At  least  the  driver  can  call  up  the  control  room  and  ask  if  they  know  what’s  going  on.  

For  our  situation,  where  a  train  is  in  the  section  ahead  and  its  full  length  has  passed  a  repeater,  a  following  train  “applying  the  rule”  should  not  be  shown  a  green  aspect.    This  is  why  repeaters  have  their  own  track  circuits.      

SCAT Unfortunately,  over  the  years,  drivers  occasionally  took  a  rather  relaxed  view  of  “extreme  caution”  and  some  nasty  rear-­‐end  shunts  took  place.    The  worst  of  these  was  in  the  tunnel  near  Stratford  (Central  Line)  on  8th  April  1953,  when  12  people  were  killed  but  there  were  a  number  of  others  both  before  and  after  this  time  and  it  was  not  until  a  spate  of  collisions  and  near  misses  in  the  1960s  and  ‘70s  that  a  system  known  as  “speed  control  after  tripping”  (SCAT)  was  introduced.    The  objective  was  to  enforce  slow  speed  on  a  train  after  it  had  been  tripped  at  a  signal.    It  appeared  on  the  1973  Tube  Stock  when  that  went  into  service  in  1975,  although  it  was  then  referred  to  as  “tripcock  delay”.    It  required  the  driver  to  proceed  at  slow  speed  whilst  an  indicator  light  was  illuminated.    It  was  retrofitted  on  refurbished  trains  (and  provided  on  new  stock)  from  the  mid-­‐1990s.  

SCAT  is  simply  a  3-­‐minute  electronic  delay  inserted  into  the  control  system  that  prevents  a  train  exceeding  (usually)  10  mph  (17km/h)  until  3  minutes  have  elapsed.    At  9  mph  the  driver  gets  

Risk Review

After the Kings Cross fire of 1987, a full review of the safety risks of all the Underground’s operations was undertaken and the possible speeds of trains after “carrying out the rule” was shown to be a high risk item. Previously, it had been recommended by Her Majesty’s Railway Inspectorate (HMRI) that the rule should be withdrawn. There had been several incidents during the 1980s which showed the rule’s vulnerability due to overspeeding – in particular, Leyton (two occasions) and Kilburn, where a driver was killed on his first day on the front alone. The rule was not withdrawn – it was simply impossible to leave a train sitting in on a line with no one knowing what was going on. However, the introduction of centralised signalling control and train radio did make it easier to communicate information.

Following the Leyton and Kilburn incidents, HMRI had recommended, in a report of 1991, that a speed limit be enforced after carrying out the rule but it wasn’t until the early 1990s that a reliable, train-mounted, time-delay system was available and a way of fitting it to older stocks had been worked out. Once it was, it was implemented as part of the safety improvements programme for refurbished trains, which included interior fire-hardening and the introduction of the PEA system in place of the old emergency stop valves.

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an  audible  alarm  and  if  he  goes  over  10  mph,  the  service  brakes  apply.    It  basically  enforces  the  “extreme  caution”  rule.    Now,  if  a  driver  isn’t  paying  attention  after  carrying  out  the  rule,  a  10  mph  bash  will  bend  a  bit  of  metal  but  it  is  unlikely  to  kill  anybody.    In  any  case,  at  that  speed,  even  the  doziest  of  drivers  is  likely  to  see  a  train  ahead  and  be  able  stop  before  hitting  it.  

One  might  ask  why  it  took  so  long  to  put  such  a  system  into  place  and  why  it  wasn’t  put  on  older  stocks  before  the  refurbishment  programme  of  the  early  1990s.    Well,  quite  simply,  the  money  wasn’t  there.    The  older  train  equipment  was  not  easily  adaptable  without  huge  expense  and  there  was  (and  still  is)  a  reluctance  to  put  anything  new  on  trains  which  might  affect  their  reliability.    

The  introduction  of  SCAT  has  produced  some  problems  of  its  own.    Now,  whenever  a  train  passes  a  signal  at  danger,  there  is  the  3-­‐minute  crawl  afterwards,  which  doesn’t  help  service  reliability.    It  also  highlights  accidental  tripping,  known  as  the  SPAD  (Signal  Passed  At  Danger),  when  a  driver  misjudges  a  signal  stop  or  suddenly  sees  a  green  signal  turn  to  red  in  front  of  him.    Largely  as  a  result  of  some  hostile  media  comment,  SPADs  are  now  regarded  as  a  safety  issue  which  needs  monitoring  by  safety  regulators,  so  they  get  reported  through  an  official  network.      

London  Underground  also  regards  SPADs  as  a  nuisance.    Trains  forced  into  SCAT  after  getting  tripped  delay  the  service  and  a  number  of  initiatives  have  been  tried  to  reduce  the  number  of  SPADs.    Crews  involved  in  such  incidents  always  get  management  attention  of  one  sort  or  another.  

Dual Aspect & Trainstop Proving A  feature  of  London  Underground  signalling  is  double  protection.    Nothing  is  ever  allowed  to  be  exposed  to  a  single  fault  which  could  render  it  unsafe.    Trainstop  operation  is  a  good  example.    A  trainstop  is  the  ultimate  protection  for  the  train.    When  a  signal  shows  a  stop  aspect,  the  trainstop  is  raised  to  trip  into  action  the  emergency  brakes  of  a  train  which  passes  the  signal.    The  trainstop  is  lowered  only  when  the  signal  shows  a  proceed  aspect.  

Now,  the  trainstop  is  raised  by  a  spring  and  held  down  by  compressed  air  pressure.    This  means  that,  if  the  air  pressure  is  lost,  even  if  the  signal  is  

clear,  the  trainstop  will  automatically  rise.    The  signal  control  circuit  detects  this  and  causes  the  red  aspect  to  appear,  even  though  the  section  ahead  is  clear  and  the  green  aspect  is  lit.    The  driver  will  then  see  both  red  and  green  together  -­‐  the  “dual  aspect”  –  which  he  will  treat  as  a  stop  signal.    He  then  applies  the  “stop  &  proceed”  rule.  

The  other  possibility  for  a  trainstop  malfunction  is  that  it  doesn’t  go  up  when  the  signal  is  red.    It  has  happened  –  suppose  an  empty  drinks  can  gets  wedged  between  the  trainstop  and  its  control  box  –  so  a  protection  circuit  called  “trainstop  proving”  is  used.    This  circuit  prevents  the  signal  in  rear  of  our  signal  from  clearing  if  our  signal  is  red  but  the  trainstop  hasn’t  gone  up.    This  is  known  as  “trainstop  proving”.    This  kind  of  protection  is  used  frequently  in  signalling  circuitry  and  is  nowadays  called  “diversity”  –  never  have  a  single  point  of  failure,  always  check  everything  twice.  

Four Aspects?

Because  you  can  only  have  one  train  in  one  section  at  one  time,  if  you  want  to  let  more  trains  through  the  system,  you  have  to  reduce  the  length  of  the  sections.    This  will  mean  that  successive  signals  will  be  closer  together.    Sometimes,  this  can  mean  that  the  signal  for  the  section  you  are  approaching  has  the  repeater  for  the  next  stop  signal  on  the  same  post.    Semaphore  signals  are  also  seen  like  this  –  the  distant  for  the  next  signal  is  mounted  below  the  previous  stop  signal.      

Fig. 10: LU signal post carrying a stop signal above the repeater for the next signal. Note the two identification plates, one white, one yellow.

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On  the  Underground,  particularly  in  open  sections,  the  repeater  and  previous  stop  signal  were  often  made  up  from  a  combined  signal  unit  with  four  aspects.    If  all  the  signals  were  clear,  the  combined  signal  would  show  two  greens  –  green  for  the  stop  signal  (always  at  the  top)  and  green  for  the  repeater  of  the  next  signal.    The  signal  head  is  as  shown  in  Fig.  10,  above.      

This  design  should  not  be  confused  with  the  4-­‐aspect  signals  used  on  the  main  line.    The  main  line  4-­‐aspect  signal  indicates  the  state  of  the  road  over  four  sections.    The  sequence  of  operation  is  red,  single  yellow,  double  yellow  and  green.    Also,  the  main  line  4-­‐aspect  signals  have  only  one  identification  plate.    The  Underground  version,  actually  being  two  signals,  has  two  number  plates,  one  in  white  for  the  stop  signal  above  another  in  yellow  for  the  repeater.      

Certain  signals  on  the  Metropolitan  Line  north  of  Harrow-­‐on–the-­‐Hill  are  genuine  4-­‐aspect  signals,  installed  to  provide  sufficient  warning  to  main  line  trains  operating  with  longer  braking  distances  than  usual  on  the  Underground.    These  areas  do  not  have  repeater  signals,  since  they  could  be  confused  with  a  multi-­‐aspect  yellow.    If  a  repeater  is  necessary  because  of  sighting  problems,  a  banner  signal  is  used.    Rickmansworth  southbound  comes  to  mind  as  one  place  which  has  a  banner  signal.      

Nowadays,  the  4-­‐aspect  look-­‐alike  stop  signal  and  repeater  combination  as  a  single  signal  head  are  not  being  installed  for  new  installations.    New  combined  stop  and  repeater  signals  consist  of  two  separate  2-­‐aspect  signal  heads,  even  if  they  are  on  the  same  post.  

In Rear and In Advance When  discussing  operations  on  a  railway,  particularly  when  you  are  dealing  with  an  emergency,  you  have  to  be  sure  you  know  where  everyone  and  everything  is.    Over  the  years,  vocabulary  has  developed  to  indicate  locations  of  trains,  signals,  structures  and  people  in  relation  to  other  objects  along  the  line.    Sometimes,  to  

the  outsider,  these  expressions  can  be  confusing.    Two  such  are  “in  rear  of”  and  “in  advance  of”.    The  following  diagram,  Fig.  11  below,  shows  what  these  two  expressions  mean.  

Train  1  is  standing  at  signal  A123.    This  is  how  the  driver  would  report  his  position  when  contacting  the  signaller.    This  is  simple  and  could  hardly  be  misunderstood.    However,  Train  2,  stopped  between  signals  A123  and  A125,  has  passed  signal  A123  but  it  has  not  reached  A125.    Although  A125  is  ahead  of  the  driver  as  he  sees  it,  he  is  said  to  be  “in  rear  of”  the  signal.    And,  although  it  has  gone  past  signal  A123,  the  train  is  said  to  be  “in  advance  of”  signal  A123.  

If  you  aren’t  paying  attention,  this  could  get  confusing.    It  is  made  even  more  confusing  by  the  appearance  in  LU  Signal  Engineering  standards  of  a  statement  which  says  that  the  stopping  point  for  a  train  “shall  normally  be  5  metres  IN  FRONT  OF  a  signal”  (my  capitals).    If  this  was  true,  i.e.  in  advance  of  the  signal,  the  train  would  have  been  tripped!    Of  course,  the  signal  engineer  is  looking  at  the  signal  the  other  way  round.    The  back  of  the  signal  is  where  he  changes  the  bulbs  and  feeds  the  wires  in.    The  front  is  where  he  cleans  the  lens.  

This  has  the  hint  of  farce  about  it  but  there  have  been  recent  instances  where  errors  have  been  made  because  of  the  misinterpretation  of  these  expressions.    With  the  huge  number  of  new  people  coming  into  the  railway  sphere  these  days,  it  is  essential  that  these  descriptions  are  standardised,  communicated  during  training  and  verified  as  clearly  understood  when  used  on  the  ground.      

Fig. 11: Diagram showing how the positions of trains are described on the London Underground.

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4 – In Control Semis So  far,  we’ve  only  looked  at  automatic  signalling  on  the  Underground.    We’ve  seen  how  each  track  is  divided  into  sections  with  a  signal  protecting  the  entrance  to  each  section  and  how,  when  a  train  runs  through,  a  track  circuit  detects  its  presence  and  replaces  the  signal  to  danger  behind  it.    As  the  train  clears  the  section,  the  signal  clears  to  green  when  the  track  circuit  picks  up.    When  talking  about  this,  it  is  said  that  the  normal  aspect  of  automatic  signals  is  green.    They  only  go  to  red  if  the  circuit  “goes  down”,  as  it  would  when  occupied  by  a  train  or  because  the  circuit  is  lost  for  some  reason.      

Automatic  signalling  is  fine  until  you  get  to  a  junction.    At  junctions,  you  need  someone  to  make  the  decisions  about  selecting  the  route  required,  which  signal  should  be  cleared  for  it  and  which  held  at  danger  to  protect  it,  so  here  a  signaller  must  be  appointed  to  do  this  and  the  signalling  is  therefore  –  to  some  extent  anyway  –  controlled  by  him.  

All  the  basic  systems  used  for  automatic  signals  we’ve  seen  above  are  retained  with  controlled  

signals  but  a  controlled  signal  needs  an  action  by  the  signaller  to  get  it  to  clear.    Because  of  this,  the  normal  aspect  of  all  controlled  signals  is  red  or  stop3.    Since  train  detection  in  controlled  areas  is  still  automatic,  using  track  circuits,  controlled  

3 Shunt signals are also controlled but most of them on LU don’t show a red light. They show a horizontal red band to indicate stop and this rotates to a 45 degree angle to indicate proceed (See Fig. 14).

signals  are  called  “semi-­‐automatic”  signals  or  “semis”,  for  short.  

Semi-­‐automatic  signals  are  identified  by  the  usual  white  enamel  plate  on  the  signal  post,  with  the  code  letters  of  the  controlling  interlocking  room  or  signal  cabin  plus  the  number  of  the  lever  (or  button)  controlling  it.    There  are  some  variations  which  we  will  see  in  due  course  as  they  arise.    Code  letters  have  changed  over  the  years,  largely  as  a  result  of  more  and  more  individual  cabins  being  closed  and  replaced  by  line  control  centres.    For  example,  the  Arnos  Grove  signal  cabin  area  used  to  be  “J”  but  was  changed  to  “PJ”  when  central  control  was  introduced  on  the  Piccadilly  Line.  

Points At  a  junction,  the  kit  that  allows  a  single  track  to  divide  into  two  is  usually  called  “a  set  of  points”  in  the  UK,  or  just  “points”  for  short.    In  the  US  they  call  them  “switches”,  which  is  ambiguous  because,  although  they  work  like  switches,  the  rails  which  move  are  called  switch  rails  and  they  “switch”  trains  from  one  track  to  another.    

Modern  ones  also  contain  switches  in  the  electrical  sense.    In  an  attempt  to  overcome  the  ambiguity,  they  are  now  often  called  “turnouts”.    We  will  stick  to  points.      

Points  comprise  a  number  of  moving  parts,  some  of  which  carry  the  weight  of  the  train  as  it  passes  over  them.    As  a  result,  they  require  careful  maintenance  and  a  stable  trackbed  upon  which  

Fig. 12: A drawing showing the basic parts of a set of points as used on conventional railways. The outside “stock” rails are fixed. The stretchers hold the moving switch rails to gauge. The points are shown in the normal position with the route set straight. A stretcher will be linked in some way to the point machine, which drives the switch rails from one position to the other. Locking and drive systems are not shown.

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to  rest  to  ensure  they  guide  the  wheels  of  the  train  safely  through4.      

Most  points  have  two  positions,  known  as  “normal”  and  “reverse”.    The  normal  position  is  usually  where  the  points  allow  the  train  to  proceed  along  the  straight  or  main  route,  while  reverse  is  for  the  diverging  route.      

Points  need  several  safeguards  to  ensure  they  operate  properly  and  safely  and  these  safeguards  have  to  be  monitored  and  linked  to  the  signals  protecting  them.    First,  the  switch  rails  –  the  rails  which  move  to  change  the  route  –  must  be  kept  to  gauge.    This  is  done  with  a  “stretcher”  –  a  bar  which  joins  the  two  switch  rails.    Often  more  than  one  is  used,  Fig  12  above.  

Next,  it  is  necessary  to  ensure  that,  once  a  route  has  been  set  up  and  the  signals  protecting  it  have  been  cleared  (placed  in  the  “off”  position),  no  other  route  can  be  set  up  that  will  conflict  with  it  and  create  the  risk  of  a  collision.    This  is  achieved  by  interlocking.    Interlocking  was  gradually  introduced  in  the  1870s  until  it  became  compulsory  under  the  1889  Regulation  of  

4 Very often, when you hear of a points failure, it is the result of poor track maintenance causing failure of the point locks or the detection system.

Railways  Act.    In  its  common  form,  interlocking  was  achieved  by  mounting  the  signal  and  point  levers  in  a  frame,  beneath  which  extensions  to  the  levers  were  interlaced  through  a  series  of  interlocking  bars.    The  bars  were  arranged  so  that,  once  a  points  lever  and  its  associated  signal  lever  had  been  reversed,  all  other  levers  which  could  be  used  to  set  up  a  conflicting  route  were  locked  so  that  the  signaller  couldn’t  reverse  them.    On  LU,  the  locking  is  achieved  by  the  use  of  small  steel  “dogs”  attached  to  the  locking  bars  which  engage  with  “crosslocks”  to  provide  the  interlocking.  

With  points,  there  was  always  the  fear  that  the  switches  could  move  as  a  train  approached  a  diverging  junction  (facing  points).    To  prevent  this  

another  lock  was  introduced.    This  was  operated  by  a  second  lever  provided  in  the  signal  box  for  each  set  of  facing  points.    This  lever  operated  what  is  called  a  “facing  point  lock”,  mounted  on  the  points  themselves.    To  change  the  points,  the  locking  lever  had  to  be  reversed  first  to  release  the  facing  point  lock,  then  the  points  lever  could  be  reversed,  after  which  the  locking  lever  was  restored  to  normal.    Then  the  protecting  signal  lever  could  be  reversed  to  show  a  clear  signal  to  the  driver.    The  whole  set  of  levers  were  

Fig. 13: Drawing of LU 4-foot, air operated point machine in a flat bottom rail layout. The drive rod is linked to the air cylinder through an escapement. The lock housing contains the lock which ensures the two switch rails are in the correct position. The detector slide extends to the PL & D (Point Lock and Detection) Box where the electrical detection is made by two detector rods being in the correct position. The whole assembly is fixed in position by the Ground Lock (always called the WL). The mechanism cannot move until the WL is released. This is unique to LU and is to ensure the points remain locked if the air supply is lost. Adapted from a Westinghouse drawing.

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interlocked  so  that  the  signaller  couldn’t  free  the  points  unless  the  locking  was  released  and  he  couldn’t  do  this  until  the  signal  lever  was  restored  to  normal.  

Even  with  this  protection,  it  would  still  possible  for  a  signaller  to  restore  all  the  signal  levers  to  normal  and  then  move  the  points  while  a  train  was  passing  over  them.    To  prevent  this,  the  locking  bar  appeared.    This  was  a  long  bar  mounted  adjacent  to  one  of  the  running  rails,  which  was  depressed  by  the  flanges  of  the  train  wheels  as  it  passed.    This  acted  on  the  facing  point  lock  to  hold  it  in  place  against  an  attempt  to  release  the  lock  by  the  signaller.  

Once  the  track  circuit  arrived,  points  could  be  electrically  protected  during  the  passage  of  a  train.    The  locking  bar  was  now  redundant.    The  occupation  of  the  track  circuit  caused  an  electrically  operated  lock,  known  as  the  “back  lock”  to  hold  the  signal  lever  against  an  attempt  to  move  it  back  it  to  the  normal  position,  thereby  maintaining  protection  of  the  route.      

As  you  might  imagine,  there  were  many  variations  on  locking  devices  but  they  all  provided  the  essential  protection  for  route  security.    All  of  them  were  adopted  by  the  Underground  in  one  form  or  another  and  eventually  they  were  incorporated  into  standard  designs  for  point  machines.      

Point Machines A  point  machine  is  the  mechanism  used  to  move  the  switch  rails  in  a  set  of  points.    Originally  they  were  operated  directly  by  the  signalman  through  rods  or  wires  connected  to  the  lever  in  his  cabin  but  both  electrical  and  pneumatic  drives  appeared  in  the  last  years  of  the  19th  century.    The  District  and  LER  tube  lines  wholeheartedly  adopted  the  pneumatic  drive  system,  imported  from  America.    Eventually  the  Underground’s  design  evolved  into  a  very  neat  and  reasonably  compact  design  which  appears  in  Fig.  13  above  and  is  known  as  the  “4-­‐foot”  machine      

The  design  is  known  as  a  4-­‐foot  machine  since  it  is  designed  to  fit  between  the  running  rails.    There  isn’t  enough  room  to  put  the  air  cylinder  outside  the  running  rails.    A  6-­‐foot  version,  where  this  is  done,  is  used  in  some  locations  where  more  space  is  available.  

London  Underground  has  used  compressed-­‐air  operated  point  machines  for  almost  100  years5.    They  have  stayed  with  them  for  two  reasons  –  (1)  they  need  a  trackside  air  supply  (see  Air  Main  in  the  box  above)  to  work  the  trainstops  so  they  might  as  well  use  it  for  point  operation  –  and  (2)  air  operated  point  operation  is  fast.    It  takes  5-­‐6  seconds  for  an  electric  point  machine  of  the  type  used  on  the  main  line  to  change  from  one  position  to  another,  (lock  to  lock,  as  it’s  called)  whereas  it  takes  only  2  seconds  for  the  LU  air  operated  points  to  change.    Suppliers  have  struggled  to  get  electric  operation  times  down  but  Westinghouse  have  said  their  new  electro-­‐hydraulic  machine  will  throw  the  points  in  “less  than  3  seconds”.  

One  of  the  best  places  to  see  air  operated  points  working  is  at  the  west  end  of  Platforms  2  and  3  at  Edgware  Road  (Met.).    You  can  see  and  hear  them  changing  from  one  position  to  another  and  you  can  see  the  signals  and  trainstops  operating  too.  

The  Underground  rationalised  point  drives  and  layouts  in  the  post-­‐war  period  to  eliminate  electric  machines  (mostly  ex-­‐Metropolitan  Railway)  and  some  earlier  LER  variants.    The  standard  arrangement  became  the  four-­‐foot  layout,  described  above.    There  were  three  sizes  of  air  cylinder  to  drive  the  escapement,  the  longer  the  turnout,  the  bigger  the  drive.    The  layout  was  available  with  and  without  a  ground-­‐lock  (Fig.  13),  which  was  always  added  to  turnouts  used  for  passenger  moves  even  though  the  escapement  itself  incorporates  a  facing  point  lock.  

One  other  “point”  to  bear  in  mind  is  that  points  are  often  provided  in  pairs  to  create  a  crossover.    The  two  ends  will  normally  operate  in  unison  from  a  single  lever  but  each  will  have  its  own  drive  and  locking  system  on  the  ground.    Points  in  a  crossover  will  have  the  same  lever  number  but  the  ends  are  lettered  A  and  B.    The  A  end  will  be  the  one  nearer  the  interlocking.   5 They also used compressed air to operate signal arms until the last of them was converted to colour lights in 1952.

4-, 6- or 10-foot?

On the railway, we always refer to the bit between the running rails as “the 4-foot”, since it is to 4ft. 8½ins. gauge. The bit between the two tracks of a normal 2-track line is called “the 6-foot” and the bit between two double track lines is called “the 10-foot”, for obvious reasons. Fortunately, no one has forced us to metricate it yet.

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Chairlocks When  the  Victoria  Line  signalling  was  being  designed,  the  signal  engineer  of  the  day  was  keen  to  develop  a  standard  layout  which  offered  true  detection  of  the  switches.    The  4ft  layout,  in  common  with  many  designs,  does  not  detect  switch  position,  it  detects  the  rods  connected  to  them,  so  if  these  are  out  of  adjustment  or  come  adrift,  the  switches  themselves  can  be  in  a  different  place  from  that  indicated.    the  Underground    wanted  to  overcome  this  and  eventually  found  a  design  developed  by  SNCF  –  the  Chairlock  (Fig.  12).    They  adapted  this  to  their  own  requirements  (the  design  had  to  be  re-­‐worked  for  Bull  Head  (BH)  rail,  of  course).      

The  chairlock  layout  is  typically  French  -­‐  elegant,  achieving  both  true  switch  detection  and  locking  and  effective  -­‐  it  has  a  very  positive  drive  but  its  

design  doesn’t  overcome  three  short-­‐comings.    First,  being  elegant,  it’s  very  difficult  to  set  up  mechanically  on  the  ground.    Once  it  is  set-­‐up  however,  it  stays  put  because  there  are  no  rods  to  expand  or  contract  or  get  walked  on  by  P-­‐way  men  etc.    The  second  problem  arises  because  the  WL  was  shoe-­‐horned  in  as  a  special  LU  requirement  and,  as  a  result,  its  tolerance  is  very  tight.    The  WL  has  to  operate  very  quickly  and  there  is  a  tendency  for  them  to  bind  on  chairlocks,  which  will  result  in  a  points  failure6.    

6 You can’t see the WL (ground lock) on a chairlock machine because it’s hidden away inside the chairlock

The  third  problem  is  that  chairlocks  are  very  intolerant  of  a  train  running  through  them.    The  closed  switch  chairlock  unit  will  be  smashed  by  a  run-­‐through  and  since  the  open  switch  unit  will  try  to  stop  the  switch  moving,  the  stretchers  usually  get  broken  too.    Fortunately,  run-­‐throughs  are  rare.  

Following  the  introduction  of  chair  locks,  someone  had  the  bright  idea  of  using  a  simpler  product  available  off-­‐the  shelf,  the  British  Rail  clamp-­‐lock.    But,  once  the  Underground’s  signal  engineer  got  hold  of  it,  it  was  no  longer  “off  the  shelf”  because  the  BR  drive  is  electro-­‐hydraulic  and  the  Underground  straight  away  converted  it  to  pneumatic  drive.    But  at  least  it  was  available  for  BH  rail.    It  is  said  that  when  you  set  them  up  with  new  switch,  stock  &  closure  rails  you  have  to  be  particularly  careful  to  get  the  detection  right,  so  they  were  little  better  than  the  chairlock.    

Nowadays,  clamp-­‐lock  spares  are  hard  to  find  and  LU  is  phasing  them  out.  

The  Underground  stuck  with  Bull  Head  rails  for  years  after  main  line  railways  started  replacing  them  with  Flat  Bottom  (FB)  rails,  largely  because  of  fears  about  clearances  in  tube  tunnels.    LU  encountered  several  problems  when  they  started  to  use  FB  point  &  crossing  layouts.    The  first  was  that  the  volume  used  on  LU  is  so  small  that  the  manufacturers  were  really  not  interested  in  developing  a  special  pneumatic  layout.    In  

unit itself and, when de-energised, physically locks that unit.

Fig. 12: Sketch of Chairlock point machine as adapted for Underground air operation. Note how the locking and detection is at the very end of the switch rail tips, making careful setting up and adjustment essential and requiring well maintained P-way to prevent detection failure. Sketch adapted from LU drawing.

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addition,  chairlocks  could  not  be  readily  adapted.    For  a  while,  LU  used  a  variant  of  the  HW1000  (as  used  on  the  Central  Line)  but  has  now  standardised  on  an  FB  version  of  the  4ft  layout  (see  Fig  11  above).      

Other  off-­‐the  shelf  machines  are  being  looked  at  for  line  upgrades,  where  another  problem  emerges.    To  meet  new  Journey  Time  Capability7  

requirements,  a  lot  of  turnouts  have  to  be  made  longer  for  higher  speeds  and  this  means  longer,  heavier  switch  rails,  which  need  more  energy  to  move  them.    Sometimes  two  point  motors  have  to  be  used.    The  limit  of  performance  of  pneumatic  drives  has  now  been  reached  and  it  is  likely  that  all  new  drives  will  be  electro-­‐hydraulic.    The  arguments  over  speed  of  drive  are  less  strong  since,  as  we’ve  seen,  modern  electro-­‐hydraulic  drives  are  quite  fast.    Also,  new  LU  signalling  designs  use  “sectional  route  release”  in  common  with  mainline  practice.    Curiously,  the  new  turnouts  at  the  Piccadilly  Junction  for  T5  are  very  long  4-­‐foot  drives  and  it  will  be  interesting  to  see  how  these  actually  perform  in  long  term  service.  

Out of Gauge Points A  certain  places,  “out  of  gauge”  points  are  provided.    These  are  usually  at  the  exit  of  a  reversing  siding  like  Colindale  or  Wood  Green.    The  points  are  arranged  in  a  “Y”  format  (Fig.  13)  and  the  two  switch  rails  operate  independently.    When  the  points  are  in  the  normal  position,  both  switch  rails  lie  away  from  the  stock  rails  so  they  are  effectively  “out  of  gauge”.    To  let  a  train  use  the  left  hand  route,  the  right  hand  switch  rail  will  close  against  the  stock  rail  and  vice  versa  for  the  right  hand  route.    The  safety  feature  of  this  

design  is  that,  if  a  train  attempts  to  leave  the  siding  against  the  signal  with  the  points  not  set,  it  will  be  tripped  (of  course)  and  it  will  derail  before  reaching  the  main  line.  

7 Part of the former PPP contractual framework.

Fig. 13: Schematic of out of gauge points. In the normal position, both the switch rails are open. Only one will close to set up a route. This arrangement provides protection for the main line against a train starting from the siding against the exit signal.

Fig. 14: Typical, externally lit, LU disc type shunt signal mounted in a tube tunnel wall with identification plate. More modern installations use an internally it LED box showing the equivalent indications. There are some locations which use BR type position light shunt signals. Photo: R Griffin.

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5 – Terminals and Timing Terminals The  design  and  use  of  signalling  in  controlled  areas  has  developed  slowly  on  the  Underground  over  the  last  100  years.    To  look  at  this  development,  we  will  take  a  simple  2-­‐track  terminus  (Fig.  15  below),  similar  to,  say,  Elephant  &  Castle  (Bakerloo)  and  see  how  it  has  evolved.    The  example  is  simplified  but  we  can  use  it  to  demonstrate  the  progressive  development  of  safety  principles8.      

In  our  layout,  trains  in  the  platforms  are  protected  by  the  home  signal  (E5)  being  set  back  a  full  speed  overlap  from  the  platform  berths.    Thus,  a  train  standing  in  either  platform  is  protected  from  an  overrunning  train  tripped  at  E5.    However  the  design  has  one  major  flaw.    If  a  train  overruns  E5  at  full  speed  while  a  train  is  leaving  Platform  1  over  No.  3  crossover,  there  is  a  risk  of  a  collision  because  it  is  within  the  overlap  of  signal  E5.    There  is  no  “flank  protection”.    

8 There is the usual health warning here for signal engineers in that you will see I have omitted some details and simplified the principles.

There  is  evidence  to  suggest  that  this  was  the  original  setup  at  least  until  the  mid-­‐1920s.      

Eventually,  it  was  realised  that  full  protection  for  a  departing  train  would  require  the  signal  to  be  moved  back  a  distance  equivalent  to  at  least  the  crossover  length.    But  this  had  a  price.    Moving  the  signal  back  increases  the  distance  to  the  platforms.    This  would  then  increase  the  time  required  for  a  train  held  to  await  a  free  platform  to  start  up  from  signal  E5  and  proceed  into  the  

station.    The  solution  was  to  ensure  that  flank  protection  was  provided  for  a  full  speed  approach  by  stopping  the  train  at  the  new  signal  position  and  then  letting  it  draw  up  closer  to  the  site  of  the  original  home  signal.    The  train  could  then  run  into  the  platform  as  soon  as  it  was  free.    Fig.  16  shows  how  this  was  done.  

Our  terminus  now  has  two  home  signals,  E5  and  E6.    The  “outer  home”,  E6,  is  positioned  so  that  a  

Fig. 15: Schematic of simple 2-track terminus equipped with a scissors crossover, a home signal (E5) and starting signals (E1 and E2) for each platform. The home signal (E5) is set back a full speed overlap from the platform berth so that if a southbound train is tripped it will stop before it could hit a train standing in either platform. The crossover is worked by two levers: Lever 3 works the points covering the exit from Platform1 while lever 4 works the points for SB moves into Platform 2.

Fig. 16: Schematic of terminus with flank protection introduced. If both platforms are occupied, signals E5 and E6 will be at danger. A SB train approaching will be tripped at E6 if it does not stop and the safe braking distance of the overlap of E6 provides “flank” protection for the crossover. To allow a train to approach E5, a timing track in rear of E6 will hold E6 at red until the train has occupied the circuit for a set time. It will then clear to allow the train to draw up to E5. Note also the Delta track on the exit of the terminus which provides “bobbing” protection as described in the text.

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train  approaching  at  full  speed  and  failing  to  stop  will  be  tripped  and  come  to  a  stand  before  it  reaches  the  crossover.    So  the  train  can  come  closer  to  the  station  ready  for  it  to  run  into  a  platform  as  soon  as  it  is  free,  a  timing  track  on  the  approach  to  E6  is  used  to  ensure  that  the  train  occupies  this  section  long  enough  to  check  it  is  not  going  to  overrun  E6  at  full  speed.    Once  this  is  proved,  by  a  15  second  timing  relay,  E6  will  clear  to  let  the  train  draw  up  to  the  “inner  home”  E5.    It  should  be  noted  that,  in  our  illustration,  if  a  train  did  overrun  E5  while  E2  was  cleared  for  a  departing  train,  the  occupation  of  the  overlap  track  would  cause  E2  to  return  to  danger.  

Next,  we  consider  a  train  departing  from  Platform  1.    The  lever  for  No.3  points  is  reversed,  the  lever  for  E2  is  then  reversed  and  the  signal  clears.    The  train  in  Platform  1  can  now  leave.    We  hope  it  will  leave  quickly  because  we  have  a  train  standing  at  E5  and  we  want  to  get  it  into  the  terminus.    However,  we  do  have  to  wait  until  the  departing  train  has  passed  through  the  crossover.    While  the  train  is  occupying  the  crossover,  its  track  circuit  prevents  the  lever  for  E2  being  restored  to  the  normal  position.    This  is  called  “backlocking”.    We  need  to  do  this  to  prevent  the  points  being  moved  under  the  train.    If  we  could  get  No.  2  lever  back  to  the  normal  position,  it  would  release  the  interlocking  on  points  lever  No.  3  and  we  could  restore  the  points  to  normal  before  the  train  was  clear  of  them.  

So  the  track  circuit  for  the  points  is  important.    However,  crossovers  are  problematic  in  that  they  can  sometimes  fail  to  detect  the  train  passing  over  them  (vibration,  bad  packing  under  the  sleepers  etc.).    This  gives  rise  to  what  is  called  a  “bobbing”  track,  where  there  is  an  intermittent  loss  of  detection.    It’s  not  common  these  days  but  it  can  happen  and  signal  engineers  are  very  cautious  people.    They  insist  on  diversity,  so  they  provide  an  additional  detection  system.    They  use  what  is  referred  to  as  a  “delta”  track  circuit.  

A  delta  circuit  is  a  10kHz  feed  into  a  running  rail  over  a  short  distance  -­‐  a  few  metres  -­‐  which  detects  the  front  of  a  train  as  it  arrives.    The  circuit  does  not  require  insulated  joints  in  the  rail  as  the  length  of  detection  is  small  and  uses  a  high  frequency  circuit.      

When  the  front  wheelset  of  the  train  is  detected  by  the  delta  circuit,  the  backlock  on  the  signal  is  energised  and  the  lever  can  be  restored  to  normal.    Now  the  points  can  be  changed  and  the  

next  SB  train  routed  into  the  terminus.    For  routes  which  involve  many  sets  of  points,  deltas  are  useful  for  allowing  the  release  of  each  set  of  points  as  it  is  cleared  by  the  train  so  that  another  route  can  be  set  up  immediately.    This  is  often  called  “sectional  release”.  

Bi-Directional Tracks. A  terminus  has  trains  running  in  both  directions,  into  and  out  of  the  platforms.    A  train  is  designed  to  operate  in  both  directions  so  it  has  a  cab  at  each  end,  with  a  tripcock  at  each  end  on  the  right  hand  side  of  the  leading  bogie  as  you  look  out  of  the  cab  end.    From  this  we  can  see  that  a  train  entering  either  platform  of  our  terminus  (Fig.15)  will  have  to  pass  over  the  trainstops  of  the  starting  signals  E1  and  E2.    As  the  train  runs  in,  the  starting  signal  will  be  at  danger.    The  driver  won’t  see  it  because  it  is  pointing  the  other  way,  ready  for  when  he  has  changed  ends  and  is  waiting  to  depart.    Since  the  signal  is  at  danger,  the  trainstop  will  be  up.    The  rear  tripcock  of  the  train,  which  will  become  the  front  tripcock  when  the  driver  changes  ends,  is  on  the  same  side  of  the  track  as  this  trainstop  and  if  it  stayed  up  as  the  train  ran  in,  the  train  would  be  tripped  on  the  rear  tripcock  –  back-­‐tripped  as  it  is  called.    To  prevent  this,  the  trainstop  automatically  lowers  as  the  train  approaches  and  then  rises  again  as  soon  as  the  train  is  clear  of  it.    This  is  known  as  “trainstop  release”.    In  order  to  operate  effectively,  trainstops  of  signals  on  bi-­‐directional  tracks  are  very  close  to  the  track  circuit  boundaries.  

Approach Locking At  a  diverging  junction,  where  the  driver  has  a  choice  of  routes,  mistakes  can  be  and  are  made.    The  commonest  is  where  the  signaller  sets  up  the  wrong  route,  clears  the  signal  for  the  train  and  the  driver  accepts  it  and  goes  the  wrong  way9.    A  favourite  place  for  this  is  Hanger  Lane  Junction  where  the  District  goes  left  towards  Ealing  Broadway  and  the  Piccadilly  goes  right  towards  North  Ealing.    Many  a  train  has  ended  up  in  the  wrong  place  over  the  years.    

9 More often than not, the mistake is not the signaller’s fault. It is usually due to a “wrong description” being sent along the line because of a cancellation, late running or out-of-turn-working.

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It  would  not  stretch  our  imagination  too  much  to  suppose  that  a  signaller,  suddenly  noticing  that  the  wrong  route  is  set  up,  would  replace  the  signal  to  danger  by  putting  the  lever  back  towards  the  normal  position  so  that  he  can  restore  the  points  to  the  position  he  wants  and  therefore  send  the  train  on  the  right  route.    However,  if  the  driver  has  got  too  close  to  the  signal  to  be  able  to  stop  before  reaching  the  points,  there  is  a  risk  the  points  could  be  moved  under  the  train.    This  possibility  is  prevented  by  “Approach  Locking”.  

Approach  locking  simply  means  that  the  signal  lever  is  locked  when  the  driver  reaches  the  sighting  point  of  the  signal.    The  track  circuit  detects  the  train  as  it  reaches  this  point  and  retains  the  backlock  on  the  lever  to  prevent  it  being  restored  to  its  normal  position.    However,  the  signaller  can  move  the  lever  a  short  distance  off  the  reverse  position  to  get  it  to  go  back  to  danger.    If  he  can  stop  the  train,  he  will.  

Once  activated,  the  approach  lock  will  be  held  until  a  time  has  elapsed  –  usually  two  minutes  –  sufficient  for  the  signaller  to  be  sure  that  the  train  is  stationary  before  making  any  changes  to  the  route.    The  signaller,  if  working  in  a  signal  cabin,  has  to  “take  a  release”  manually  by  a  hand-­‐operated  screw  device  or  a  dedicated  lever  operating  a  time  delay  relay.    In  other  installations,  the  time  release  is  automatic.    The  driver,  having  seen  the  signal  “go  back  in  his  face”  as  they  say,  will  have  got  in  touch  by  now  anyway.  

The  approach  track  to  a  junction  signal  is  also  often  used  to  clear  the  signal  when  the  train  has  reached  a  certain  point  in  rear  of  the  signal.    Although  the  route  is  set,  the  signal  will  not  clear  until  the  train  has  occupied  the  approach  track  circuit.    This  has  the  effect  of  causing  the  driver  to  pay  attention  to  the  signal  showing  red  so  that,  when  it  changes  to  green,  he  will  check  to  see  that  the  route  is  the  correct  one  for  his  train.    Well,  this  is  the  theory  anyway.    WM21/22  at  Hanger  Lane  Junction  is  like  this  but  it  hasn’t  prevented  generations  of  drivers  from  accepting  the  wrong  signal,  with  Piccadilly  trains  going  round  to  Ealing  Broadway  and  Districts  to  North  Ealing.    The  same  setup  on  an  approach  track  also  has  the  effect  of  causing  the  driver  to  reduce  speed  where  a  diverging  route  requires  a  speed  limit.  

Timing Circuits Certain  routes  require  train  speeds  to  be  reduced  to  “preserve  the  integrity  of  the  train/track  interface”  –  in  other  words,  to  prevent  the  train  flying  off  the  tracks.    Traditionally,  drivers  are  shown  a  speed  limit  sign  which,  traditionally,  they  usually  adhere  to,  more  or  less,  but  which  they  can  ignore.    To  enforce  speed  limits,  the  signal  engineer  introduced  certain  devices  which  persuade  the  driver  to  reduce  speed,  or  at  least  to  pay  attention.    The  approach  controlled  signal  at  Hanger  Lane  Junction  mentioned  above  is  one  way  of  forcing  a  driver  to  reduce  his  speed  as  he  approaches  a  junction.    The  approach  to  Watford  South  Junction  used  to  be  like  this  but  drivers  knew  the  setup  many  ignored  it.    They  knew  that  when  the  train  reached  a  certain  block  joint,  the  signal  would  clear.    In  five  years  of  approaching  it,  it  almost  always  cleared.    On  the  few  occasions  it  didn’t  I  got  the  brakes  on  and  stopped  at  the  signal  –  just.    But  then,  I  knew  I  could  since  I  had  already  adjusted  the  train  speed  to  match  the  possibility.  

This  approach  didn’t  work  for  everyone.    The  in-­‐town  part  of  the  Central  Line  used  to  have  several  stations  where  there  were  three  home  signals  on  the  approach.    The  outer  home  was  approach  controlled.    If  there  was  a  train  in  the  platform,  all  three  would  show  red.    When  the  driver  saw  them,  he  would  slow  down  and,  when  he  occupied  the  approach  track  circuit,  the  outer  home  would  clear,  even  if  the  train  ahead  was  still  in  the  platform.    There  was  no  time  delay.    The  idea  was  that,  since  the  driver  had  slowed  his  train,  he  could  approach  the  next  signal  at  the  lower  speed  and  still  have  a  safe  braking  distance  if  he  got  tripped.  

One  driver,  approaching  Holborn  WB  on  9th  July  1980,  where  such  an  installation  was  in  place,  knew  the  setup  and  assumed  that  the  train  ahead  would  leave  as  he  approached.    It  had  always  done  so  before  so  why  not  this  time.    He  approached  at  full  speed.    As  he  entered  the  approach  track,  the  first  signal  cleared.    He  expected  the  next  one  to  clear  as  the  train  in  front  left  the  platform.    But  it  didn’t  and  he  got  tripped  at  close  to  full  speed.    The  overlap  on  this  second  signal  was  not  designed  for  a  full  speed  trip  and  the  train  ran  through  to  the  rear  of  the  one  in  the  platform  and  collided  with  it  at  about  12mph.    It  turned  out  that  the  train  in  the  

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platform  was  delayed  while  the  crew  were  attending  to  a  sticky  door.      

Luckily,  no  one  was  killed  but  a  valuable  lesson  was  learned.    If  you  devise  a  system  which  relies  on  the  integrity  of  a  wide  variety  of  users,  eventually  one  of  them  will  find  a  way  to  abuse  it.    As  a  result,  approach  control  of  this  type  on  Central  Line  outer  home  signals  was  quickly  removed.    Other  similar  sites  along  the  line  already  had  timers  on  the  approach  track,  which  retained  the  red  aspect  for  4½  seconds  to  ensure  that  the  train  would  be  tripped  if  it  didn’t  reduce  speed.  

Terminal Protection Another  accident,  some  5  years  earlier  on  28th  February  1975  at  Moorgate,  also  arose  as  a  direct  result  of  driver  error  and  also  led  to  changes  in  the  signalling  system.  The  changes  became  known  as  terminal  protection.    This  accident  was  a  lot  more  serious  than  the  one  at  Holborn  and  led  to  the  deaths  of  42  passengers  and  the  driver.    The  circumstances  were  simple;  the  train  ran  through  the  station  at  35mph  and  hit  the  end  of  the  tunnel.    The  driver  never  made  any  attempt  to  stop  and  the  guard  didn’t  have  time  to  react  to  stop  the  train.  

Soon  afterwards,  the  Underground  began  experiments  with  various  forms  of  terminal  

protection.    The  eventual  result  was  that  all  dead  end  platforms  and  reversing  sidings  were  fitted  with  timing  sections  which  forced  a  train  to  slow  down  during  its  approach  to  the  buffer  stops.    Typically,  the  home  signal  has  a  timing  section  to  make  the  driver  reduce  the  train  speed  to  20mph  and  there  are  two  “blind  trainstops”  in  the  platform  or  siding.    A  blind  trainstop  is  one  without  a  signal.  

At  the  entrance  to  the  platform,  the  first  blind  trainstop  remains  raised  until  the  timing  section  detects  the  train  speed  is  18mph  or  less  and,  30  metres  in  rear  of  the  stopping  mark,  there  is  a  second  trainstop  which  requires  12mph  or  less  before  it  will  lower.    There  is  also  a  fixed  trainstop  at  the  stopping  point.    Drivers  usually  end  up  watching  the  trainstops  rather  than  the  train  speedo,  since  the  latter  is  regarded  as  less  reliable.  

One  serious  outcome  of  the  terminal  and  siding  protection  scheme  has  been  a  reduction  in  line  capacity.    Terminals  are  often  the  limiting  factor  for  the  capacity  of  a  whole  line.    Nowhere  is  this  more  obvious  than  Aldgate  where,  because  of  terminal  protection,  8-­‐car  A  Stock  trains  have  to  creep  across  the  junction  into  either  of  the  two  bay  platforms,  effectively  blocking  the  route  for  Hammersmith  trains.    On  other  lines,  some  reversing  sidings  are  now  virtually  disused  as  a  

result  –  Wood  Green  being  a  good  example.    The  delay  cause  by  the  combination  of  detraining  a  terminating  train  and  then  waiting  for  it  to  creep  into  the  siding  just  became  unacceptable.    Without  Wood  Green  reversers,  more  trains  had  to  be  sent  to  Cockfosters  to  reverse  but,  of  course,  they  have  to  negotiate  the  terminal  protection  there.    Nowadays,  as  soon  as  there  is  a  small  delay  to  the  Piccadilly  Line  service,  congestion  at  Cockfosters  can  become  a  serious  problem.    This,  combined  with  other  factors  like  

Fig. 17: Schematic of a layout where a conflicting route is protected by a draw-up signal. The overlap of the starter (E12) extends over the junction so that the junction is not fully flank protected if a train gets tripped at E12. The draw-up signal E120 is positioned in rear of E12 so that its overlap provides full protection. A train approaching the platform will brake before it reaches the platform so that it enters the timing track at reduced speed. A 4½s timer will clear E120 to yellow, lowering the trainstop to allow the train to draw up to E12.

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defensive  driving10  and  one  person  operation,  have  led  to  a  reduction  in  Piccadilly  Line  capacity  of  20%.      

Draw-Up Signals There  are  certain  places  on  the  Underground  where  two  tracks  converge  or  cross  just  beyond  a  station  platform.    The  best  known  is  perhaps  Baker  Street,  where  the  double-­‐track  junction  occurs  just  east  of  the  station  platforms.    The  overlaps  of  the  starting  signals  of  Platforms  5  and  3  actually  extend  over  the  junction,  meaning  that,  if  a  train  got  tripped  at  speed  at  either  signal,  it  would  not  stop  before  it  reached  the  junction  and  could  collide  with  another  train  which  was  passing  on  the  conflicting  route.    To  overcome  this  problem,  “Draw-­‐Up  Signals”  were  introduced.    Originally,  they  were  called  “Permissive  Signals”  but  the  name  fell  into  disrepute  during  the  late  1960s  because  the  term  “permissive  society”  became  common  and  the  Underground,  it  seems,  was  anxious  not  to  be  associated  with  perceptions  regarding  the  lowering  of  public  morals.  

A  draw-­‐up  signal  (Fig.  17  below)  is  a  3-­‐aspect  signal  positioned  in  rear  of  the  starting  signal  so  that  the  driver  will  see  it  as  he  approaches  the  platform.    The  three  aspects  are  red,  yellow  and  green.    If  the  starting  signal  is  “off”,  the  draw-­‐up  signal  will  also  be  green.    If  the  starting  signal  is  red,  the  draw-­‐up  signal  will  also  be  red.    As  he  approaches  the  signal,  the  driver  will  reduce  his  speed  and  a  timing  track  will  detect  the  train  and  will  allow  the  signal  to  clear  to  a  yellow  aspect,  lowering  its  trainstop  before  the  train  reaches  it  if  the  speed  is  low  enough.    The  draw-­‐up  signal  is  identified  by  the  letter  and  number  of  the  signal  operating  with  it,  plus  a  zero  or  two  as  necessary  to  bring  it  up  to  three  digits.    

Round the Bend? I  have  already  mentioned  the  “stop  and  proceed”  rule,  where  a  driver,  confronted  by  a  red  signal  

10 Defensive driving is where drivers are taught to drive cautiously, creeping into station and crawling up to signals. It could be regarded it as a waste of the infrastructure, a restriction on line capacity and a down-grading of the skill of the driver, who is relatively well paid and is therefore an expensive resource which should be optimised. Better training is the answer. Eventually, of course, ATO will take the driver out of the equation.

which  fails  to  clear  after  a  set  time  will,  if  he  can,  seek  permission  to  pass  it  and  then  proceed  under  extreme  caution  into  the  section  ahead.    With  automatic  signals,  the  driver  can  proceed  without  permission  if  he  can’t  get  it.    As  I’ve  mentioned  before,  some  drivers  have  taken  a  rather  relaxed  view  of  “extreme  caution”  and  have  gone  too  fast  to  allow  them  to  stop  in  time  when  they  see  a  train  ahead  of  them.    In  some  cases  this  resulted  in  deaths.    Many  cases  involved  trains  negotiating  curves  in  tunnels  or  in  places  where  sight  lines  were  poor.    To  help  reduce  these  problems,  additional  stop  signals  were  inserted  in  certain  locations.    Originally  they  were  called  “Stratford  Signals”,  after  the  Stratford  accident  on  8th  April  1953  where  a  driver  carried  out  the  rule  at  a  rather  brisk  speed  and  caused  a  collision  which  resulted  in  the  deaths  of  12  people.    Later  they  became  known  as  “Round-­‐the-­‐Bend  signals”.      

Round-­‐the-­‐Bend  (RTB)  signals  are  only  installed  in  tube  tunnel  sections  where  the  curve  radius  is  less  than  300m  and  only  in  automatically  signalled  areas.    They  do  not  affect  the  normal  headway  and  a  driver  would  not  usually  see  them  at  danger.    Of  course,  if  a  driver  has  passed  the  previous  signal  under  the  rule,  he  would  have  to  repeat  the  process  when  stopping  at  the  RTB  signal.      

More  recently,  in  some  places  where  signal  sighting  is  poor,  countdown  markers  have  been  provided  on  the  approach  side  (in  rear  of)  the  stop  signal.    I  suspect  these  have  been  introduced  as  a  cheaper  alternative  to  RTB  signals.  

Modern Interlocking Earlier,  we  looked  at  interlocking  for  junctions,  both  in  the  lever  frame  and  on  the  ground  where  the  points  are.    These  systems  were  entirely  mechanical  but,  over  many  years,  interlocking  for  railway  signalling  eventually  progressed  from  mechanical  systems  to  relay  systems  and  more  recently  to  microprocessors  and  computers.    Although  the  main  line  railways  had  started  using  relay  interlocking  as  early  as  1929,  London  Underground  stuck  with  mechanical  interlocking  for  signal  frames  until  the  1990s.    Relays  had  been  used  for  remote  operation  of  sites  like  Wood  Green  but  the  safety  was  still  enshrined  in  the  mechanical  lever  frame.    It  wasn’t  until  the  resignalling  of  the  Central  Line  in  the  early  1990s  

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that  the  Underground  saw  the  first  full  relay  interlocking.    It  was  introduced  at  West  Ruislip  in  1991  and  several  more  sites  were  converted  as  the  resignalling  moved  eastwards.    Later  installations  had  Computer  Based  Interlockings  (CBI)  instead  of  relays  and  some  of  the  relay  installations  were  later  converted  to  CBIs  as  well.    

As  you  would  imagine,  relay  interlocking  uses  sets  of  relays  in  place  of  mechanical  frames  to  prevent  the  setting  up  of  conflicting  routes.    The  main  line  railways’  progress  in  this  direction  was  driven  by  the  growing  need  for  huge  mechanical  frames  at  complex  areas  like  London  Bridge,  where  a  311  lever  frame  weighing  23  tons  was  required.    Relays  considerably  reduced  the  weight  and  space  required  at  such  places.    The  Underground  generally  had  smaller  installations  and,  for  a  long  time,  it  was  the  policy  to  keep  sites  small  so  as  to  reduce  the  impact  of  failures.    With  the  Underground’s  smaller  mechanical  frames,  the  need  for  relays  was  seen  as  less  urgent.    

Of  course,  relays  bring  their  own  problems,  not  the  least  being  the  need  to  have  all  vital  (safety)  circuits  duplicated  in  such  a  way  that  a  single  wrong  side  failure  could  not  set  up  an  unsafe  condition.    For  larger  installations,  thousands  of  relays  were  needed  and  the  circuitry  was  complex.    Design  checking  and  circuit  testing  were  very  time  consuming.    Perhaps  it  was  because  the  Central  Line  resignalling  of  the  early  1990s  was  carried  out  by  contractors  (Westinghouse)  and  they  held  a  large  proportion  of  the  risks  for  getting  the  safety  systems  right  that  London  Underground  were  prepared  to  let  them  adopt  relay  interlocking.      

The  first  conversion,  at  West  Ruislip,  was  not  a  happy  story.    It  was  decided  to  re-­‐signal  the  area  and  replace  the  track  at  the  same  time.    Also,  the  signalling  had  to  be  set  up  for  the  new  ATO  system  but  trainstops  and  visual  signals  had  to  be  retained  for  the  old  1962  Tube  Stock  which  was  still  using  the  line.    This  was  the  first  contractor  led  signalling  installation  on  the  Underground  and  there  was  a  lot  of  friction  between  the  Underground  signals  people,  who  were  used  to  doing  their  own  thing,  and  Westinghouse  who,  not  having  done  it  before  on  LU,  were  actually  learning  installation  as  they  went  along.      

The  result  was  painful.    The  original  plan  was  to  close  West  Ruislip  for  a  month  and  terminate  the  service  at  Ruislip  Gardens.    Those  of  us  who  

remembered  the  4-­‐day  Easter  weekend  resignalling  changeover  by  British  Rail  at  the  huge  terminus  at  Liverpool  Street  were  horrified  and  complained  that  we  would  be  the  laughing  stock  of  the  industry  if  we  closed  the  little  two-­‐platform  terminus  at  West  Ruislip  for  a  month  just  for  resignalling.    Eventually  the  project  team  settled  for  9  days  over  the  August  Bank  Holiday  weekend  and  up  to  the  following  weekend.    With  friction  between  the  teams,  some  wrong-­‐side  failures  during  testing,  changes  to  engineering  management  and  safety  requirements  resulting  from  the  Hidden  Report11  and  the  newness  of  the  contractual  arrangements,  there  were  constant  delays  and  stoppages  of  work.    The  resulting  chaos  meant  that  West  Ruislip  did  not  open  again  until  9th  December  1991.  

In  the  late  20th  century,  railways  woke  up  to  the  fact  that  computers,  or  parts  of  them  like  micro-­‐processors,  could  be  used  in  signalling.    As  with  relays,  there  was  a  cautious  approach  at  first,  although  as  early  as  1974,  LU  tried  one  at  Rickmansworth  for  the  remote  control  of  Watford,  retaining  the  mechanical  interlocking  frame  for  safety.    By  the  mid  1980s,  electronics  for  safety  systems  were  being  considered  in  the  form  of  vital  processing  of  interlocking  (VPI)  on  the  main  line  railways  and  LU  followed  in  1987  when  they  had  such  a  system  installed  at  Northolt  as  a  trial,  working  in  parallel  with  the  existing  mechanical  frame.  

Of  course  development  was  slow.    People  were  rightly  nervous  about  using  computers  for  safety  systems.    It  was  considered  essential  that  some  sort  of  real-­‐time  checking  was  in  place  in  case  the  processor  integrity  failed,  the  programme  corrupted  itself  or  the  original  programme  code  contained  unforeseen  bugs.    This  led  to  the  employment  of  multiple  computer  systems  using  two-­‐out-­‐of-­‐two  or  two-­‐out-­‐of-­‐three  voting  systems  before  a  route  could  be  cleared.    Another  checking  technique  is  to  use  two  or  more  processors  with  different  individual  logic  programmes  within  the  computer  to  maintain  the  diversity  requirements  of  the  vital  systems.    On  main  line  railways,  processor  logic  was  developed  for  SSI  (Solid  State  Interlocking),  first  tried  at  Leamington  Spa  in  1985  and  this  evolved  into  CBI   11 “Hidden Report” - Investigation into the Clapham Junction Accident of 12 December 1988 by Anthony Hidden QC, in which a number of recommendations regarding resignalling projects were made and adopted by both BR and London Underground.

22

(Computer  Based  Interlocking)  as  later  used  on  the  Underground.    In  all  of  these  systems,  relays  are  still  used  for  the  interfaces  with  the  trackside  equipment.  

Remote Securing If,  when  a  route  is  set  up,  any  of  the  associated  point  detection  or  locking  systems  fail,  the  signal(s)  protecting  those  points  will  also  “fail”.    In  reality,  they  are  not  failing,  they  are  providing  the  protection  for  which  they  were  designed  and  are  therefore  functioning  correctly.    However,  this  is  no  consolation  for  the  passengers  stuck  on  a  stationary  train  because  of  a  “points  failure”  or  for  the  control  room  staff  trying  to  keep  the  trains  moving.  

Sometimes,  the  points  will  set  and  lock  but  the  signal  still  fails  to  clear.    One  way  of  allowing  trains  to  move  under  these  conditions  is  to  secure  the  points  and  then  authorise  the  train  to  proceed  slowly  past  the  red  signal  –  the  “stop  and  proceed”  rule  described  briefly  earlier.    Until  about  15  years  ago,  points  had  to  be  secured  manually.    Someone  from  the  nearest  station  –  usually  the  supervisor  –  had  to  walk  to  the  points  carrying  a  “clip  and  scotch”.    He  would  fit  the  clip  to  hold  the  closed  switch  rail  to  the  stock  rail  and  insert  the  scotch  to  keep  the  other  switch  rail  open.    The  driver  would  then  be  authorised  to  proceed  cautiously  through  the  route.  

Of  course,  we  are  assuming  that  the  points  are  in  the  correct  position  and  do  not  require  the  services  of  a  signal  technician  to  come  down  and  “blow  them  over”  to  the  correct  position.    All  this  takes  time  –  often  20  minutes  to  half  an  hour  –  until  a  way  of  securing  points  remotely  was  introduced12.  

Remote  securing  is  operated  by  the  signaller  in  the  control  room.    He  has  a  special  button  which  

12 The original purpose of remote securing was to allow a train through a route to rescue a stalled train blocking that route. It was soon realised that it could be used to overcome signal problems too.

effectively  adds  a  lock  to  the  points  to  hold  them  in  place  and  to  prevent  any  changes.    If  the  points  are  detected  correctly  and  the  lock  works,  a  “Route  Secure”  sign  will  be  displayed  at  the  signal.    The  driver  can  now  accept  an  instruction  to  proceed  slowly  into  the  route.  

Route Proving You  will  recall  that  earlier  I  mentioned  that  it  is  common  for  a  pair  of  points  to  be  used  to  provide  a  crossover  and  for  these  points  to  work  together  as  a  set  (Fig.  18).    Since  all  the  vital  protection  in  such  an  arrangement  is  interlocked,  it  follows  that  once  the  points  are  reversed  for  a  crossing  move  and  the  signals  for  the  chosen  route  provide  a  proceed  indication,  the  whole  crossover  is  safe  for  a  train  to  pass  over  it.    The  same  will  apply  for  the  route  when  the  crossover  is  in  the  normal  position  and  the  two  directions  are  being  used  as  normal.    In  recent  years,  this  feature  has  been  used  as  a  way  of  overcoming  certain  types  of  signal  failure.    It  is  known  as  “route  proving”.      

Another,  similar  system,  using  a  “route  card”  is  also  used.    At  each  location  where  a  suitable  route  can  be  proved,  a  “route  card”  is  provided.    This  shows  a  special  procedure  which  must  be  carried  out  to  prove  a  route  and  allow  a  train  to  move  over  a  set  of  points  against  a  red  signal  and  without  the  points  being  scotched  and  clipped.  

Fig. 18: Schematic of route proving using a trailing crossover. In this example, signal E5 has failed to clear, detaining Train 7. In order to prove the points are locked, Signal E1 will be used to bring Train 1 to a stand and the driver will be told to wait even when the signal clears to green. Train 7 is then authorised to pass signal E5 at danger and proceed “under rule”. During this manoeuvre, the driver of Train 1 monitors E1 to ensure the green aspect is maintained. The purpose of the operation is to remove the need to manually secure the points on the crossover.