Automatic Train Control in Rail Rapid Transit (Part 5 of 18)

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Chapter 3

AUTOMATIC TRAIN CONTROL

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Train control is the process by which the move-ment of rail rapid transit vehicles is regulated forthe purposes of safety and efficiency. The process iscarried out by a combination of elements-somemen, some machines—located on the train, alongthe track, in stations, and at remote centralfacilities. These elements interact to form a com-mand and control system with four major func-tions:

. Train Protection prevention of col-lisions and derail-ments,

. Train Operation con t ro l o f t r a inm o v e m e n t a n dstopping at sta-tions,

● Train Supervision direction of trainmovement in rela-tion to schedule,

. Communica t ion interchange of in-formation amongthe elements of thesystem.

The train control system is analogous to the sen-sory organs and central nervous system of thehuman body. It senses and processes information,makes decisions, and transmits commands. Also asin the human body, the execution of commands isnot a function of the train control system but ofother parts specialized for that purpose. For exam-ple, the train control system may sense train speed,determine that it should be increased, provide anappropriate command signal to the motors, andmonitor to see that the desired result is achieved.The means by which a speed change is effected,however, are not part of the train control system.All the equipment for getting electric power to thewayside, bringing it into the train, converting it tomechanical energy, and providing tractive effort isexternal to the train control system. Similarly, theequipment to select a route for a particular train andtransmit commands to aline switches accordinglyare within the train control system, but the parts ofthe trackwork that actually move (the switchpoints) are not elements of the train control system.

TRAIN CONTROL

SYSTEM FUNCTIONS

Presented below is a description of the specificfunctions performed by a train control system and

of the way in which functional elements interact.These functional relationships are also illustratedby the diagram in figure 1. Since the purpose is onlyto provide the reader with a general background forunderstanding the nature of train control, thedefinitions presented here are brief and nontechni-cal.8

Train Protection

Train protection is a family of functions whosepurpose is to assure the safety of train movement bypreventing collisions and derailments. 9 rain pro-tection functions and requirements override allother control system functions either throughequipment design or, in a completely manual mode,by rules and procedures. The functions that makeup train protection are:

Train detection—monitoring of the track todetermine the presence and location of trains;

Train separation-assuring that trains on thesame track maintain a safe following distanceto prevent collisions;

Route interlocking—preventing trains on cross-ing, merging, or branching routes from makingconflicting (unsafe) moves that would cause acollision or derailment;

Overspeed protect ion—assuring that t rainspeed remains at or below the commanded orposted civil speed limit10 as to prevent colli-sions resulting from going too fast to stopwithin the available distance and to prevent=

derailments due to excessive speed on curvesor through switches;

Train and track surveillance-observing condi-tions on and in the vicinity of the track aheadof the train and monitoring safety-related con-ditions on board the train.

Train Operation

Train operation consists of those functionsnecessary to move the train and to stop it at stations

F. For more detailed technical descriptions of train controlsystem functions and technology, see appendices A and B.

gThere is no unive~]ly accepted terminology and scheme ofdefinitions for train control system functions within the transitindustry. The terms and classification employed here are basedon several sources and represent the best of current usage.

IOA g]os5ary of train control and rail rapid transit terms is

provided in appendix D.

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PROTECTION

I PERFORMANCEMODIFICATION

k

r I

(TrainPresence)

1

I 1

I L IA

Movement Order

I 1

Movement Report

Operation

OPERATION

J I

“ To simplify the diagram, the functions of Alarmlng and Recordkeepinq are not shown

to board and discharge passengers. Train move-ment, as controlled by train operation functions, isunder the direction of train supervisory functionsand always within the constraints of train protec-tion functions. Train operation involves the follow-ing:

Speed regulation-controlling train speed, with-in the constraints of overspeed protection, tomake the run according to schedule;ll

Station stopping—bringing the train to a stopwithin some specified area in a station;

Door control--opening of doors in stations topermit passengers to enter or leave the trainand closing of doors when the train is ready tostart ; l2

Train starting—initiating train departure from astation after the doors are closed (and pro-vided the train protection system permits it),13

Train Supervision

Train supervision involves monitoring the move-ment of individual trains in relation to schedule androute assignments and overseeing the general dis-position of vehicles and flow of traffic for thesystem as a whole. The train supervision systemmay thus be thought of as making strategic deci-sions which the train operation system carries out

11Speed regulation involves more than matching actual tocommand speed. It also includes control of acceleration, jerklimiting (controlling the rate of change of acceleration), slip-slide control (correction of wheel spinning during accelerationand skidding during braking), and flare-out (gradual relaxationof braking effort as the train comes to a stop). Flare-out is con-sidered by some transit engineers to be a subsidiary function ofspeed regulation, and hence part of the train control system. Ac-celeration control, jerk limiting, and slip-slide control areregarded by transit engineers to be propulsion and brakingsystem functions, but they are mentioned here because of theirrelationship to the train control functions of speed regulationand station stopping.

12The mechanisms that actually open and close doors are notpart of the train control system, but the signals to actuate thesemechanisms and the interlocks to assure that doors are closedbefore starting and that they remain closed while the train is inmotion are generated within the train control system. Because ofthe safety implications of door control, some transit engineersconsider it to be a part of train protection.

13Train starting is sometimes classified as part of the doorcontrol function. It is separated here for two reasons: (1) in someautomated systems, door control is automatic while train start-ing is retained as a manual function; (2) in manual systems, thedoor control and train starting functions are often assigned todifferent persons.

tactically, In addition, train supervision includescertain information processing and recording ac-tivities not directly concerned with train safety andmovement but necessary to the general scheme ofoperations. Train supervision functions are:

Schedule design and implementation—prepar-ing a plan of service in light of expecteddemand, available equipment, and environ-mental conditions and issuing a schedule toimplement the plan;

Route assignment and control--selecting andassigning routes to be followed by trains (andrerouting as necessary);

Train dispatching-controlling train departuresfrom terminals or waypoints in accordancewith the schedule;

P e r f o r m a n c e m o n i t o r i n g — f o l l o w i n g t h eprogress of trains against the schedule by ob-taining periodic updates of train identity, loca-tion, and destination;

Performance modification—adjusting move-ment commands and revising the schedule inresponse to train, traffic, and environmentalconditions.

Alarms and malfunction recording-alerting tomalfunctions, breakdowns, or problems, andrecording their time, location, and nature;

Recordkeeping —maintaining operational logsand records for business and payroll purposes,for scheduling maintenance, for ordering sup-plies and equipment, and for computing tech-nical statistics.

Communication

The communication system is the means bywhich the information needed to carry out all othertrain control functions is transmitted and ex-c h a n g e d . l4 This information may take any ofseveral forms-voice, visual, auditory, and digital

14On the function diagram in figure 1, communication func-tions are indicated by the lines connecting the boxes whichrepresent train protection, operation, and supervision functions,

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or analog electrical signals.l5Unlike other train con-trol functions, which involve information process-ing and decisionmaking, communication is largely afacilitative process-serving to convey informationbut without producing any unique functional out-comes of and by itself. For this reason, thecategorization given below indicates not functionsas such but major classes of information that mustflow throughout the system in order for other traincontrol functions to take place:

Train protect ion—information necessary tolocate individual trains, to assure their safeseparation, to prevent overspeed, and to con-trol movement at route interlockings;l6

Command and s ta tus—information on theoperational state of the system, command sig-nals to control train and switch movement,and feedback to determine the response ofsystem elements to command inputs;l6

Emergency—information on the nature andlocation of emergency events and summonsfor help to elements within the transit systemor to outside agencies (e.g., fire, police, medi-cal, and rescue);

Passenger service—information relating to trainservice and system operation for the purposeof assisting passengers using transit facilities;

Maintenance—information needed to plan orconduct preventive and corrective mainte-nance;

Business operations--operational informationused to maintain a record of (and to plan for)work force allocation, vehicle utilization, pro-curement of supplies and equipment, operat-ing expenses, and system patronage.

15Some transit engineers limit the definition of communica-tion to verbal or visual communication (radio, telephone, TV,and the like). Machine-to-machine communications, since theytend to be very specialized, are considered part of the functionwhich they serve. This seems to be unnecessarily restrictive andmakes an artificial distinction between information exchange byhuman operators and other forms of information exchange in-volved in operating the system (i.e., man to machine or machineto machine). The definition offered here is generic and embracesall types of information flow, regardless of how effected.

16Customarily, this part of the communication system is com-pletely separate from the network used for other types of infor-mation and is considered to be an integral part of the train pro-tection system.

AUTOMATION

At one time or another, all of the train controlfunctions listed above have been performed byhuman operators, and many still are, even in themost technologically advanced transit systems.Theoretically, any of these functions could also beperformed by automatic devices, and more andmore have, in fact, been assigned to machines overthe years, Before examining the technology bywhich train control automation has been achieved,it is first necessary to consider what is meant byautomation and to clarify the terminology used inthis report.

Figure 2 is a generalized diagram of the processby which any train control funct ion is ac-complished. It involves receiving information aboutsome operational state of the system and somedesired state. This information must then be in-terpreted—for example, by comparing the twostates and deriving a quantitative expression of thedifference, Next, an appropriate control response tonull the difference must be selected, and somespecific command message to the controlled ele-ment must be formulated and transmitted. A final,and all-important, step is monitoring the results ofthe control action to ascertain that the desiredsystem state or condition has been achieved. Thislast step, called feedback, provides an input signalto start the process all over again, thereby creating aloop that permits the control process to be con-tinuous and adaptive.l7

If all of the steps in the general sequence shownin Figure 2 are performed by a human operator, theprocess is called manual, even though manual ac-tion in the strict sense may not be involved. Thus,manual denotes a process that may include visual,auditory, and other forms of sensory perception aswell as purely cognitive activities such as in-terpretation, weighing alternatives, and decision-making. The command output might be ac-complished by some manual activity such as press-ing a button or moving a control lever, or it mighttake the form of a voice command or simply a nodof the head. The essential feature of a manual proc-ess, as the term is used here, is that all the basic con-trol steps to accomplish a function are human ac-tivities.

l~his cIescriptiOn overlooks the difference between closed-and open-loop control systems. For a discussion of the applica-tion of each in train control technology, see appendix B.

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It is also possible for all of the steps in the controlloop to be accomplished by some mechanical orelectrical device. If so, the process is called auto-mated. The device need not necessarily be compli-cated, nor is a computer required in order for the ap-paratus to process information and make a “deci-sion.” A simple junction box with a two-state logiccircuit (ON or OFF) would satisfy the definition ofan automated control device, provided no humanactions were required to receive and interpret inputsignals, select and order a response, and monitor theresult.

Between the extremes of purely manual controland fully automatic control, there are numerouscombinations of mixed man-machine control loops.These are called semi-automated or partially auto-mated—the terms are used synonymously to denotea process (or a system) in which there are bothmanual and automatic elements. Thus, automationis not to be taken in an absolute, all-or-nothingsense. The machine can be introduced by degreesinto a system to perform specific functions or partsof functions. When comparing parts of a train con-trol system or when comparing one system withanother, it is therefore possible to speak of automa-tion in comparative terms and to say that one ismore or less automated than another, depending onhow many specific functions are performed bymachines.

For brevity, acronyms are used to describe cer-tain areas where automation is applied in train con-trol. ATC (automatic train control) refers generallyto the use of machines to accomplish train control

functions. It does not necessarily suggest a com-pletely automated system. It can be applied to asystem where certain functions or groups of func-tions are performed automatically while others areperformed manually. ATP (automatic train protec-tion), ATO (automatic train operation), and ATS(automatic train supervision) are used to designatemajor groups of functions that may be automated.For example, if a system is said to have ATP, itmeans that train protection is accomplished (eithercompletely or mostly) by automatic devices withoutdirect human involvement. If a system is describedas having ATC consisting of ATP and some ATS,this indicates that train protection is assured byautomatic devices and that train supervision is amixture of manual and automatic elements. By im-plication, train operation in such a system would bemanual.

While automation involves the substitution ofmachine for human control, this does not mean thatthe human operator is removed from the systemaltogether. An automated system is not always anunmanned system, even though all functions areroutinely performed by machines. For instance,train protection and train operation may be com-pletely automatic in a given transit system, butthere could still be an operator or attendant onboard the train to oversee equipment operation and,most importantly, to intervene in the event offailure or malfunction. This emergency and backuprole is, in fact, a major type of human involvementin even the most automated train control systems,In all rail rapid transit train control systems now in

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operation or under development, automation isutilized only for normal modes of operation, withmanual backup as the alternative for unusual condi-tions, breakdowns, and emergencies.

In passing, it should also be noted that automa-tion is not synonymous with remote control, eventhough the two may at times go hand in hand. Intrain supervision, for example, many functions areaccomplished manually by controllers who arephysically far removed from the train and wayside.In central control facilities, the operators may neveractually see the vehicles or track and yet perform allor most of the functions necessary to set up routes,dispatch trains, and monitor traffic. Conversely,automated functions are often performed locally,i.e., by devices on board the train or at a station orswitch. In general, the location of the controllingelement in relation to the controlled element is in-dependent of how the functions are accomplished.However, it is also true that automation does facili-tate the process of remote control, and systems witha high level of ATC tend also to employ morecentralized forms of train control, especially forsupervisory functions.

AUTOMATIC TRAIN

CONTROL TECHNOLOGY

The automatic equipment that accomplishestrain control functions is often of complex design,but the basic technology is quite simple. The pur-pose of this section is to provide an acquaintancewith the fundamental e lements of an ATCsystem—track circuits, signaling apparatus, trainoperating devices, interlocking controls, and super-visory equipment, The details of this technologyand the design features of ATC equipment now inuse in rail rapid transit systems are omitted here butare provided in appendices B and C.

Track Circuits

For safety and efficient operation of a transitsystem, it is imperative to know the locations oftrains at all times. The sensing device providing thisinformation is the track circuit, which was inventedover 100 years ago and has remained essentiallyunchanged in principle even though extensivelyrefined and modified in its engineering details.

FIGURE 3.—Simple D.C. Track Circuit

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The track circuit is an electrical circuit consistingof a power source, the running rails, and a signalr ece ive r ( r e l ay ) .l8 The track is divided intoelectrically isolated segments (called blocks) by in-sulated joints placed at intervals in the runningrails. l9 This forms a circuit with a power sourceconnected to the rails at one end of the block and arelay at the other. The relay, in turn, forms part of asecond electrical circuit which has its own indepen-dent power supply (commonly a battery) and in-cludes a signaling device such as wayside coloredlights,

When no train occupies the block, the relay isenergized by the track circuit battery, causing therelay to “pick up, ” i.e., a movable element (ar-mature) is moved to and held electromagnetically ina position opposed to the force of gravity. Thiscloses an electrical contact in the secondary signalcircuit. When a train enters the block, the wheelsand axles conduct electricity between the runningrails, thereby short circuiting (shunting) the trackcircuit and reducing the current to the relay. Thisweakens the electromagnetic force holding up thearmature, allowing it to drop under the force ofgravity. This action opens the contact that was pre-viously closed and closes a different contact in thesignal circuit. The relay, therefore, acts as a switchin the secondary signal circuit and creates one

electrical path when it picks up and another when itdrops.

Thus, the basic principle of the track circuit is theshunt ing phenomenon produced by the t ra inwheels passing along the electrically energized run-ning rails. The presence of the train is detected inthe track circuit as a reduction of electrical current,which-by means of the relay—is used to controlthe secondary signal circuit and operate varioustypes of track occupancy indicators.

The track circuit is designed according to the fail-safe principle. In order for a clear (unoccupiedblock) indication to be given, the track circuit mustbe in proper working order. If one of the rails were

18Track circuits may utilize one or both running rails, mayoperate on direct or alternating current, and may haveelectromechanical relays or solid-state electronic receivers. Thetype described here is a double-rail dc track circuit with a relay.The other types are similar in principle and operation.

19Block length in rail rapid transit systems varies considera-bly as a function of track and traffic conditions and signalsystem design. Some are as short as 40 feet; others are over half amile long,

to break, the relay would receive no current; andthe armature would drop just as if a train were pres-ent. A broken electrical connection, a failure of thepower source, or a burned-out relay coil would alsohave the same effect.

Wayside Signals

One of the earliest types of signal devicesemployed to control train movement, and one stillwidely used, is the automatic wayside block signal,It consists of a color-light signal, in appearancemuch like the traffic signal on city streets, locatedbeside the track at the entrance to each block, Thissignal is controlled by the track circuit relay, asdescribed above. The signal directs train movementby displaying red, yellow, or green lights (aspects)to indicate track circuit occupancy ahead,

Since it would be impractical for the train tocreep ahead block by block, waiting to be sure eachblock is clear before entering, the wayside signalsare arranged to give the operator advanced indica-tion of speed and stopping commands. Figure 4 is anillustration of a three-block, three-aspect waysidesignal system, This signaling arrangement tells thetrain operator the occupancy of the track threeblocks ahead of the train and conveys threedifferent movement commands (indications)—green (proceed), yellow (proceed prepared to stop atthe next signal), red (stop).

In the illustration, Train A is stopped in Block 4and Train B is approaching from the rear. Sincethere is a separation of at least three blocks betweenthem, Train B receives a green aspect at theentrance to Block 1, allowing it to proceed at themaximum allowable speed. At the entrance toBlock Z, however, Train B receives a yellow aspect,indicating that the train operator should be pre-pared to stop at the next signal because there maybe a train ahead. At the entrance to Block 3, Train Bis commanded to stop by a red signal aspect. WhenTrain A leaves Block 4 and moves on to Blocks 5and 6, the signal at the entrance to Block 3 changesto yellow and then green, allowing Train B to pro-ceed.

The wayside signaling system is made fail-safethrough design and by operating rules. Dual, orsometimes triple, lamps are used to illuminate eachsignal aspect. Redundant power sources are some-times provided. The ultimate safeguard, however, is

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FIGURE 4.—Three-Block, Three-Aspect Wayside Signal System

procedural. A complete failure of the signal lampsor a loss of power would result in a dark (unlighted)signal, which standard operating rules require thetrain operator to observe as if it were a red signal.

Trip Stops

In the wayside signal system described above,safe train movement depends solely on the com-pliance of the operator with signal indications. Toguard against error, inattention, or incapacitation ofthe train operator, wayside signals can be supple-mented with an automatic stop-enforcing mecha-nism, called a trip stop.

The trip stop is a device located beside the trackat each wayside signal. The type commonly used inthe United States consists of a mechanical arm thatis raised or lowered in response to the track occu-pancy detected by the track circuit. When the arm isin the raised position, it engages a triggering deviceon the train and actuates (trips) the emergency

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brake.20 A train entering a block in violation of thewayside signal indication would thus be brought toa complete stop before colliding with the train inthe next block regardless of what action the trainoperator took, or

In addition tosions, trip stops

failed to take.

protecting against rear-end colli-can also be used in conjunction

with the track circuits and other signal appliances toprovide automatic protection against overspeed. Forthis application, a timing device is added to the cir-cuit controlling the trip stop. When a train enters a

20An alternative system employing inductive train stops is

used on main-line railroads in the United States and on railrapid transit systems abroad. The device is somewhat more com-plex than the mechanical trip stop, but it avoids mechanical con-tact between a stationary wayside element and a moving trainand is less vulnerable to blockage by snow or debris. Both tripstops and inductive train stops have the inherent disadvantageof requiring strict alinement of wayside devices. Further, ifeither type of device is removed, the system will operate in amode that is not fail-safe.

block, the trip stop at the entrance to the next blockis in the raised position but will be lowered after atime interval corresponding to the minimum time(the maximum speed) permitted for a train to tra-verse the block. This arrangement is commonlyused on curves, downgrades, and other such sec-tions of track where excessive speed could cause aderailment. A variation of this scheme is commonlyused at stations to allow a following train to close inon a leading train, provided the follower moves atappropriately diminishing speed as it approaches itsleader.

Like track circuits and signals, the trip stop isdesigned to operate in a fail-safe manner. The trip israised to the stopping position by gravity or a heavyspring and lowered by a pneumatic or electricmechanism. Thus, failure of the trip stop actuatingmechanism or its source of energy will result in thetrip stop being raised to the stop position.

Cab Signals

Automatic block signal systems with waysidesignals and trip stops, while offering effective trainprotection, have certain operational disadvantages.Sometimes the signals are obscured by fog, rain, orsnow. In such cases, operating rules require that theoperator consider the signal as displaying its mostrestrictive aspect and operate the train accordingly.If the signal is actually displaying a more per-missive indication, time is lost unnecessarily. A sec-ond disadvantage is that wayside signals conveycommands only at the entrance to a block. The trainoperator must reduce speed to the maximum per-mitted by the signal and maintain that speed untilreaching the next signal. If conditions change im-mediately after the train enters the block and itbecomes safe to proceed at a greater speed, the trainoperator has no way of knowing this since the sig-nal is behind him. Again, time is lost. With waysideblock signals there is also the possibility that theoperator will fail to observe the signal correctly,read the wrong signal in multiple-track territory, orforget the indication of the last signal passed. Ifthere are trip stops, these kinds of human failure donot result in an unsafe condition, but the efficiencyof train operation can be adversely affected.

One way to overcome these disadvantages is toprovide signal displays within the cab of the train,This is called cab signaling, A display unit, mountedin the cab within the train operator’s forward field

of view, shows indicator lights similar to those ofwayside signals, e.g., red, yellow, and green aspects.Cab signals can thus convey the same movementcommands as wayside signals, but they do so con-tinuously in response to the instantaneous condi-tion of the track ahead. They can also convey pre-cise speed commands instead of just stop-and-go in-formation, thus providing more flexible operationand paving the way to ATO. The cab signal unit hasan audible warning that sounds whenever the sig-nal aspect becomes more restrictive and continuesto sound until the operator silences it by anacknowledging device. Figure 5 is an illustration ofa typical cab signal.

Transferring the display of information from thewayside to the cab involves an alternate type oftrack circuit technology. To operate cab signals, thecurrent passing through the track circuit (usuallya.c. is not steady, as for conventional wayside sig-nals, but is pulsed (turned on and off) at severaldifferent repetition rates in response to track occu-pancy. Each pulse rate is a code to indicate allowa-ble train speed. This pulsed d.c. energy is passedthrough the rails, picked up inductively by areceiver (antenna) on the train, and decoded toretrieve speed command information, This infor-mation is used to actuate the appropriate cab signaldisplay. Because the train is continuously receivingpulses of energy, a change in the pulse rate of thecoded track circuits indicating a change of condi-tions ahead of the train is instantaneously receivedby carborne equipment and displayed by cab signalsregardless of where the train happens to be within ablock.

Figure 6 illustrates how cab signals control atrain in a three-block, three-aspect signalingsystem. In this example, the code rates transmittedthrough the rails (expressed as pulses per minute)correspond to the following signal aspects:

180 Green (Proceed)

75 Yellow (Proceed at medium speedprepared to stop)

O Red (stop)21

21Note that O code-the absence of a code—is the mostrestrictive, Thus, any failure of the track circuit or the carbornereceiver is a fail-safe condition since it is interpreted by the cabsignal equipment as a command to stop.

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HOW IT WORKS: Receiver coils, mounted on the train near the rails, receive pulse-coded track signals, whichare decoded and used to pick up relays that energize the cab signal lamp indicating trackconditions ahead.

FIGURE 5.-Cab Signals

FIGURE 6.—Three-Block, Three-Aspect Cab Signal System

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The situation depicted here is the same as in the il-lustration of wayside signals (figure 4). Train B isapproaching Train A, which is completely stopped,Note that the moment Train A starts to move andclears the block, Train B receives a green signal im-mediately—not at the entrance to the next block, asit would with wayside signals. Note also that a Ocode appears in the part of the block immediatelybehind Train B as it moves along the track and thatTrain B can approachrequired to stop,

Speed Control

closer to Train A before being

With the addition of speed sensing and brakecontrol mechanisms, cab signals can also be used toprovide automatic overspeed protection. Figure 7 isa schematic diagram of such a system. It is the sameas the schematic shown in figure 5, except for theaddition of speed and code rate comparison equip-ment and the direct connections to the propulsionand braking systems.

This arrangement allows the train operator tocontrol speed so long as it does not exceed the com-manded speed shown on the cab signal unit. If thecommanded speed is exceeded or if the block speedchanges to a lower value because of another trainahead, the operator receives an audible warning.The operator has a fixed time (typically 2 to 3 sec-onds) to initiate the required braking manually. Ifthis is done, the brakes can be released when thecommanded lower speed is reached. If not, thebrakes are applied automatically and irrevocably bythe ATC system, and the train is brought to a fullstop before the operator can resume control. This isanalogous to the overspeed control provided bywayside signals with trip stops, except that brakingcan be initiated anywhere within a block not just atthe entrance. Another difference is that trip stopsact to stop the train after an overspeed condition hasoccurred over a measured course, usually severalhundred feet in length. Cab signals do the same, butinstantaneously, thus eliminating the delay in-herent in the preliminary measured course and per-

Train Wheels

/

& Axle

FIGURE 7.—Cab Signal System With Automatic Overspeed Protection

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mit trains to follow one another more closely for agiven block length.

Automatic Train Operation

Basically cab signaling provides carborneautomatic train protection in the form of collisionprevention. With the addition of on-board equip-merit for sensing and comparing command (allowa-ble) and actual speed, cab signaling makes it possi-ble to expand the train protection function to permitspeed regulation. This, in turn, forms the basis forextending automation into the area of train opera-tion.

Several forms of automatic train operation(ATO) are possible, but all have two basicfeatures-automatic speed regulation and stationstopping.

Automatic speed regulation (ASR), as the nameimplies, is basically a comparator circuit for match-ing actual speed to command speed. Speed comandsreceived from coded track circuits are picked up bya carborne receiver, decoded, and compared to ac-tual train speed sensed by a tachometer in the drivemechanism. Up to this point, an automatic speedregulation system is like cab signaling. Thedifference arises in how this comparison is used.With cab signals, the comparison is used to actuatea penalty brake application to stop the train whenactual speed exceeds command speed. With ASR,the comparison is used to control the motors andbrakes in an effort to minimize the difference be-tween actual and command speed. An advisory dis-play of speed commands and train speed may beprovided for the operator. In effect, ASR removesthe human operator from the control loop for run-ning the train and provides for an essentially instan-taneous and invariant response by propulsion andbraking systems, without the delay of human reac-tion time and without the variability and possibilityfor misinterpretation inherent in manual trainoperation.

The other basic element of ATO is station stop-ping, which involves bringing the train to stopautomatically at a predetermined location in eachstation. This is accomplished by special waysidecontrol units working in cooperation with positionreceivers, logic circuits, and automatic speedregulation equipment on the train. One methoduses wayside “triggers” spaced some distance fromthe station as reference points for programed stop-ping. The first trigger, farthest from the station,

transmits a command signal thatboard the train, a velocity-distance

generates, onprofile which

the train is to follow to a stop. Additional triggers,nearer the station platform, correct the generatedvelocity-distance profile for the effects of wheelslip and slide. The ASR system monitors thevelocity-distance profile and controls the brakingeffort to bring the train to a stop at a predeterminedpoint. Another method of programed stoppingmakes use of long wayside antenna to provide aseries of position signals to a carborne controlsystem as the train passes along its length. The car-borne control system determines train position andcombines this with speed and deceleration informa-tion (sensed on board the train), to produce an ap-propriate propulsion or braking command for thetraction control system.

To this basic ATO system, other automatedfeatures may be added. Doors can be openedautomatically after the train is brought to a stop in astation, This requires a circuit to actuate door open-ing mechanisms and appropriate safety interlocks toassure that the train is in fact stopped and at a sta-tion. Door closure may also be automated by addinga timing circuit to measure how long the doors havebeen open and to initiate a door closure signalautomatically after a predetermined dwell time haselapsed. Train departure can also be initiatedautomatically by introducing another control circuitto apply propulsion power after receipt of a signalconfirming that doors are closed and locked.

For each of these levels of ATO, the train opera-tor may be provided with an advisory display toshow what commands are being received and whatresponse is being made by automatic mechanisms.The operator may also be provided with manualoverride controls to inhibit automatic functions orto vary automatic system operation. For example,the operator may intervene manually to adjust thestopping point, to prevent some or all doors fromopening, to vary station dwell time, or to initiate orprevent departure. Figure 8 shows a functionaldiagram of a typical ATO system and a picture ofthe train operator’s console.

Interlocking

An interlocking is an arrangement of signals andsignal appliances so interconnected that functionsmust succeed each other in a predetermined se-quence, thus permitting safe train movements alonga selected route without collision or derailment. An

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FIGURE 8.—Automatic Train Operation System

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FIGURE 9.—Typical Interlocking Location

interlocking thus consists of more than justswitches to allow trains to move along crossing,merging or branching routes; it is also made up ofsignals and control devices that automatically pre-vent conflicting or improper movements. Interlock-ing may be manually controlled or equipped withautomatic devices that sort trains through branchesand junctions according to desired destinations,

Several forms of automatic interlocking are inuse. One of the oldest and simplest is an arrange-ment of hand-operated switches, each of whichcontrols an individual signal or track turnout. Theswitches are mechanically or electrically intercon-nected such that once a particular route is selected,the switch points locked in place, and the signalscleared, no other route for a potentially conflictingmove can be established until the train bound forthe cleared route has safely passed, This arrange-ment represents a semiautomated form of move-ment control, Manual operation is required to selecta route and move the control levers, but all else

follows automatically, including inhibition offurther switch movement until the train has tra-versed the limits of the interlocking.

A more advanced, but still not completely auto-mated, type of interlocking is a system that permitsa towerman or central supervisor to select theentrance and exit points for a train to pass throughan interlocking, with the switches and signals fort h e a p p r o p r i a t e r o u t e t h e n b e i n g s e t u pautomatically by an arrangement of electricalrelays, Figure 10 shows such a control panel for asystem called entrance-exit route interlocking, Thetower operator moves the control knobs to desig-nate a desired route. Internal logic circuitsautomatically select the best available nonconflict-ing route, aline and lock switches, and activate theappropriate wayside signals to allowing train move-ment while holding other signals at stop to preventconflicting moves. This level of automation may becharacterized as automatic execution in response tomanual inputs,

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FIGURE 10.—Entrance-Exit Interlocking Control Panel

Fully automatic interlocking are also in use. Inaddition to track circuits, switch operation, and sig-nal control elements, the automatic interlockingmust have some device for identifying a specifictrain in order to create the necessary input to thelogic circuits.22 One method to identify trains is bymeans of wayside optical device that scans a panelon the lead car which gives destination, route, andother needed information. Another method makesuse of a carborne transponder that is interrogated bya wayside device. With either technique, however,train identity becomes the substitute for manual in-puts that allows trains to be sent along predeter-mined routes without human involvement.

22A rudimentary form of automatic interlocking is one thatuses a simple in-out logic circuit to switch trains from one trackto another. This device is commonly used at terminals and oper-ates to switch each entering train from the inbound to the out-bound track and thus does not require train identity information.

Train Supervision Equipment

Train supervision embraces a wide variety offunctions. The special-purpose equipment that hasbeen developed to perform these functions isequally varied. In a general survey of train controltechnology it is not possible to describe all types ofautomatic and semiautomatic devices that are inuse. The following, therefore, is a brief catalog ofsome of the more important systems.

Train dispatching is concerned with the timing oftrain departures from terminals in accordance withthe schedule of operations. In conventional transitsystems this function is accomplished bypreprogrammed dispatching machines thatautomatically ring a bell or flash a light as a signalto the train operator that it is time to leave a ter-minal or intermediate waypoint. In some systems,the dispatch function may be assigned to a centraltrain control computer that transmits electric start-

31

ing signals to the train in accordance with a masterschedule stored in the computer memory.

Route assignment and control is a train super-visory function that is allied to the train protectionfunction of route interlocking. Route control is astrategic function, consisting of selecting routes fortrains and transmitting the orders to wayside points,where the orders are implemented tactically by in-terlocking equipment. In conventional transitsystems, route assignment and control is performedlocally, either manually or automatically, Withremotely controlled route interlocking, however, itbecomes operationally practical to place thestrategic and tactical management of routing in acomputer. The programing to accomplish this isrelatively simple and straightforward, and a com-puter is ideally suited to handle what is an essen-tially repetitious task with a limited number ofalternative courses of action. The safety aspects ofroute interlocking are assured not by central com-puter control, but locally by conventional interlock-ing equipment at the wayside,

Performance monitoring involves comparing theoverall movement of traffic with the schedule and

taking action to smooth out irregularities of trafficflow. In most transit systems this function is carriedout by central control personnel aided by automaticdisplay devices. One such device is a pen recorderthat marks a moving paper graph to record thepassage of trains past check points, Each spike onthe graph indicates the presence of a train, asdetected by the track circuits, at some time andplace along the route. A train supervisor, by check-ing this graph against the schedule, can monitor theprogress of all trains operating on line and detectdelays or queuing up of trains, (Figure 18, page 36

shows such a device, )

Another form of performance monitoring aid isthe model board (figure 11), which is a schematicrepresentation of the track plan of the transitsystem with indicator lights to denote track circuitoccupancy and, hence, the position of each train onthe line, This is the functional equivalent of thepengraph recorder, but in a more pictorial form ofdisplay. Another type of model board used in newertransit systems has, in addition to the master trackplan, small cathode ray tube displays that permit in-dividual supervisors to obtain more detailed or ex-panded views of selected track sectionsspecial-purpose presentations of data,

or to call up

.

FIGURE Il.—Model Board and Train Control Console

32

Pengraphs, model boards, and the like are notfully automatic supervisory devices. The humanoperator is still needed to interpret the display andto formulate orders to individual trains. In the mostadvanced systems routine performance monitoringis assigned to computers, which keep a continuouswatch on traffic movement and automaticallycalculate and transmit performance commands totrains. Man, in this circumstance, acts in a com-pletely different supervisory capacity. He does notmonitor and regulate traffic. Instead, he supervisesmachines which, in turn, monitor and regulatetraffic.

There are two general types of action that can betaken to smooth out irregularities in traffic flow.Both are accomplished in response to commandsfrom central control. One is to hold a train in a sta-tion for a time longer or shorter than the scheduleddwell time or, in extreme cases, to direct a train tobypass a station in order to close up a gap. The othermethod is to alter the speed of the train betweenstations. This latter method is called performancelevel modification and takes the form of a propor-tional reduction of train speed below the speed nor-mally allowed in each block. In systems supervisedby a central computer and with automatic trainoperation, performance level modification is ac-complished without human intervention. The re-quired reduction is calculated by the central com-puter and automatically transmitted to stations orother critical locations, where the signals are pickedup by carborne ATO equipment that modifies theresponse to the normal speed commands transmit-ted by the coded track circuits. These systems may

also include provisions for manual inputs and dis-plays at central control or on the train, but the nor-mal mode of operation is automatic.

A WALK

THROUGH A TRANSIT SYSTEM

To place ATC in perspective, it maybe helpful tomake a brief tour of the facilities of a transit system,pointing out the type and location of the equipmentthat carries out train control functions.

Station

The passenger’s first point of contact with a tran-sit system is the station. The most prominentfeatures-vending and fare collection facilities(possibly automated), escalators and elevators,heat ing and air condi t ioning, and platformamenities—have nothing to do with train control.There may also be public address systems and videoor audio surveillance equipment for fare collectionand platforms. These are not, strictly speaking, partof the train control facility even though they maybeconnected to the central control facility andmonitored by central supervisory personnel, Aboutthe only direct manifestations of ATC are the auto-mated train departure and destination signs orloudspeakers found in some transit systems. Thesepublic announcement devices are connected to theATC system and use information inputs derivedfrom track circuits and train identification equip-ment, There may be an ATC equipment room in thestation, but it is out of sight and locked. Its presenceis usually unknown to passengers.

FIGURE 12.—General View of Rail Rapid Transit Station

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—-——

These are the impedance bonds that isolate thetrack into blocks. At the ends of the blocks, thereare small boxes, containing relays, with electricalconnections from the track circuits to the signalingapparatus,

FIGURE 14,—Track Circuit Wiring

Other signal equipment is contained in smallcases placed at intervals along the right-of-way.There are also telephones or other communicationequipment and antennas or transmitters used forprecision station stopping, train identification, orperformance level modification, In certain loca-tions, ATC apparatus and other trackside equip-ment may be housed in small sheds to protect theequipment from the weather and to facilitate main-tenance by wayside workers.

FIGURE 13.—Trip Stop

Wayside

An observant passenger might notice twowayside features that can be seen from the stationplatform. Looking down the tracks in the directionof train movement, there are wayside signal lightsthat change aspect from time to time. Often, justbeyond the downstream end of the platform andalongside the rail, there is a trip stop which can beseen to raise behind a train that has just left the sta-tion and later lower as the train recedes.

Moving out along the tracks, other wayside ele-ments can be found. The track circuits themselvesare not plainly visible since they are largely inwayside housings. However, at intervals there aresmall flat equipment cases situated between therails and connected to them by electrical wiring.

FIGURE 15.—Wayside Equipment Case (contains multiplex

equipment for transmitting information between trains andstation control rooms)

At junctions and crossovers there is switch ap-paratus, the most visible parts of which are theswitch points, frogs, levers, and motive equipment,This is the wayside equipment, known as a switchmachine, that performs the function of interlockingfor train protection,

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FIGURE 16.—Track Apparatus at an Interlocking

By far the largest par t of the equipment , train but along the wayside and in central controlfacilities, and structures along the right-of-way— facilities.trackage, tunnels, bridges, the third rail, and powerdistribution equipment-are not related to train Central Controlcontrol. Nevertheless, the wayside is where thebulk of the ATC equipment in a transit system is lo- Supervisory control of the system may be exer-cated. The proportion varies as a function of the cised in a central control room equipped with modellevel of automation, but generally about 80 percent boa rds , communica t i on , equ ipmen t , sy s t emor more of all train control equipment is not on the monitoring apparatus, and individual supervisor’s

FIGURE 17.-Central Train Control Facility

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FIGURE 18.—Two Views of a Central Control Facilitywith Electromechanical Equipment

left-a clock-driven paper tape device for dispatching trains

above-pengraph device for monitoring train movement

consoles. If the system has ATS, the computers andother data processing equipment are also located inthe central control building, which often houses ad-ministrative and training facilities as well.

Not all transit systems have a single centralizedcontrol facility. Some disperse control and supervi-sion to outlying towers, situated at major interlock-ing along the routes. Figure 19 is a photograph ofsuch a local control tower.

FIGURE 19.—Tower for Local Control of Interlocking

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Vehicles

Most of the ATC equipment on transit vehicles iscarried in equipment cases under the body or in thetrain operator’s cab. About the only featuresdistinguishable from outside the train are a receivercoil mounted on the lead car to pick up coded trackcircuit signals (figure 20) and—for systems with op-tical scanners-small identification panels mountedon the side of each car.

FIGURE 21.—Train Operator’s Console for Systemwith ATO

The operator’s cab contains the displays and con-trols necessary to operate the train or to monitor thefunctions of ATC equipment. The amount andsophistication of this equipment varies greatly—ranging from very simple and utilitarian apparatusin manually operated systems to highly complexconsoles in the newest and most automatedsystems, The console typically includes propulsionand brake controls, a speedometer and commandspeed indicator, lighted placards indicating theoperating state of automatic elements, warninglights, pushbuttons or control knobs to make datainputs or to select various operating modes, a trainphone or radio for communicating with centralsupervisors, a passenger address microphone, and adeadman control to prevent the train from operat-ing in case the operator is inattentive or incapaci-tated.

Yards and Shops

A large part of the important activity of a transitsystem does not occur in revenue service on themain lines, but in the yards and shops, Thesefacilities, though seldom seen by the riding public,contribute greatly to the quality and level of servicethat the transit system offers.

The yards are usually located near terminals andconsist of a vast complex of tracks for storing vehi-cles and making up trains to be operated on thelines. Even in systems with the most advancedlevels of automation, train operation in yards isunder manual control. Train sorting and classifica-tion is also an essentially manual operation,although some systems have a limited amount ofautomatic switching in the yards, principally to andfrom revenue tracks.

Car shops and maintenance facilities are usuallylocated within the yard complex. The shops containfacilities for light and heavy maintenance, compo-nent repair, car washing, and checkout of vehiclesbefore they are dispatched back into service,

The maintenance facility may also include a test

track and special test equipment to qualify vehiclesand components for acceptance or to carry out trialsof equipment modifications.

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——

FIGURE 22.—Aerial View of Rail Rapid Transit Yard and Maintenance Facility

38

39

——

LEVELS OF AUTOMATIONIt was suggested earlier that train control

automation can be viewed as a continuum. At oneextreme, all functions are performed by humanoperators; at the other, all are performed bymachines. The transit systems now in operation orunder development in this country lie at variouspoints between these extremes, with their relativepositions corresponding roughly to the age of thesystem. The older systems generally have the

LEVEL

Essentially Manual

Wayside Signal Protection

.

Carborne Train Protection

Automatic Train Operation

Automatic Train Supervision

Unmanned Operation

Full Automation

lowest levels of automation—primarily ATP withsome ATS. The newer systems have ATP and ATOand more extensive ATS, None are completelyautomated.

Historically, the conversion from manual toautomatic train control in rail rapid transit has beenincremental and has followed a more or less com-mon course for all systems. These major technologi-cal stops along the road to automation are outlinedbriefly below and summarized in table 1.

TABLE I.—Levels of Automation

CHARACTERISTICS

Train protection by rules and proceduresTrain operation manual (with or without the aid of

advisory wayside signals)Train supervision by towermen and/or central dis-

patched

Wayside block signals with trip stops for trainseparation and overspeed protection

Train operation manualSupervision manual with some automation of dis-

patching and route interlocking

Cab signals and equipment-enforced train protectionTrain operation manualSupervision as above

Automatic Train Protection as aboveTrain operation either completely automatic or with

manual door operation and train startingTrain supervision as above

ATP and ATO as aboveTrain supervision automatic

central computer control

ATP, ATO, ATS as aboveNo on-board operator

(or mostly so) under

System manned only by small number of centralcontrol personnel

ATP, ATO, ATS as above, with automatic, notmanual backups for each

Skeleton force at central controlYard operation automated

EXAMPLE

CTA(Ravenswood andEvanston Lines)

NYCTA

CTA

PATCO

BART

AIRTRANS

None

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Essentially Manual

At this level, train protection, operation, andsupervision are carried out by train operators andtowermen or central supervisors with little or no aidfrom automatic equipment. Trains are protectedand operated either by rules and procedures aloneor with the aid of advisory wayside signals. Thereare no automatic stop-enforcing mechanisms eitheron the wayside or on board the train. Train dis-patching is carried out by personnel at terminals orat control towers along the routes, using either awritten schedule or timing devices that act asprompters to signal train departure. Route assign-ment and interlocking control are accomplished bymanually activated equipment that may have someautomatic safety features but are entirely controlledby human operators. Communications are by meansof visual signals (lights, hand signals, posted civilspeeds, etc. ) or by telephone from stations andtowers to central control.

Many of the older transit systems in this countrybegan operation at the manual level, but they havesince advanced to more automated forms of traincontrol. One of the last vestiges of a purely manualsystem is on the Ravenswood and Evanston lines ofthe Chicago Transit Authority, which as late as1975 operated without any automatic block signalprotection,

Wayside Train Protection

Wayside signals with trip stops form the basis forautomatic train protection, by assuring separationof following trains and preventing conflictingmoves at interlocking. Incorporation of timingdevices with the trip stops also provides equipment-enforced overspeed prevention. While train protec-tion thus becomes automatic, train operation is stillcompletely manual. Train supervision also remainsan essentially manual activity, although track cir-cuits and signals used primarily for train protectiondo permit some automation of route interlockingand dispatching—usually in the form of semi-automatic devices (i. e., manually activated butautomatically operating).

All t r a n s i t s y s t e m s i n t h e U n i t e d S t a t e s h a v e a t

least this level of automation. The most notable ex-

a m p l e o f a n e n t i r e s y s t e m w i t h e n f o r c e d w a y s i d e

s i g n a l i n g i s t h e N e w Y o r k C i t y T r a n s i t A u t h o r i t y .

P o r t i o n s o f t h e C h i c a g o , B o s t o n , a n d C l e v e l a n d

systems and all of the Philadelphia (SEPTA) systemalso employ this form of automatic train protection.

Carborne Train Protection

Cab signaling, using coded track circuits andautomatic carborne stopping and speed limit en-forcement, represents the same level of ATP aswayside signals with trip stops. To this extent, thislevel of automation is equivalent to the preceding.Generally, however, cab signaling is considered ahigher level of automation since it also providessome automatic aids to train operation—principallyautomatic and continuous display of speed informa-tion to assist the operator in running the train andstopping at stations. Other aspects of train operationare still essentially manual. Cab signaling does notnecessarily lead to any increase in the automationof supervisory function nor is it accompanied byany change in the communications systems.

This level and form of automation is generalIyregarded as the minimum for a new transit system,and most of the older transit systems either haveconverted or plan to convert to cab-signaled ATP.

Automatic Train Operation

The major advantage of cab signaling overwayside signaling is that bringing the speed com-mand on board the train also permits evolution toautomatic train operation. All of the informationneeded to operate the train automatically is eitherinherent in the cab signal system or readily availa-ble through modular additions. At this level, thehuman is removed from the speed control, stationstopping, door control, or starting loops--or anycombination of them. The human no longer func-tions as an operator but as an overseer of carbornecontrol systems.

Along with ATO, there is of ten (but notnecessarily) an increase in the level of automationof train supervisory functions. ATS functions thatare sometimes considered operationally desirable toimplement at the time ATO is installed includeautomatic dispatching, route assignment, and per-formance level modification.

The two newest transit systems in this country—Bay Area Rapid Transit and the Port AuthorityTransit Corporation —both have ATO. The newsystems under development in Washington, Atlan-ta, and Baltimore will also have it.

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Automatic Train Supervision

Train supervision functions (except for dispatch-ing and route control) are among the last to be auto-mated. To be effective and operationally practical,ATS usually can be introduced only when there is ahigh level of automation in the areas of ATP andATO.23 Automatic train supervision also requires arather complex and sophisticated communicationnetwork, not only for voice messages but also forthe interchange of large quantities of data amongautomatic system elements on a real-time basis.The distinguishing feature of ATS, however, is theuse of a central computer (or computers) to processand handle data, make decisions, and formulate in-structions.

The Bay Area Rapid Transit system was the firstrail rapid transit system to make extensive use ofATS. The new Washington, Atlanta, and Baltimoresystems will also have highly automated trainsupervision based on computer control. While thereare some differences among them in the type andamount of control vested in ATS computers, thesefour stand apart from all other transit systems inthis country in the extent to which automationtechnology is applied to train supervision.

Unmanned Operation

At all the levels of automation described pre-viously, there is at least one operator on board eachtrain and some supervisory personnel in centralcontrol. While these people are not part of the nor-mal control loop, they do exercise important func-tions as overseers of automatic equipment andback-ups in case of failure or emergency; A moreadvanced form of automation is one where thetrains are unmanned, with all ATP and ATO func-tions performed by automatic devices. The few re-maining human operators in the system are atcentral control, but even these personnel may be

23Even with ATP and ATO, ATS is not truly necessary untilthe demands imposed by the complexity of the route structureand the required level of service outstrip the capacity for effec-tive real-time supervision by manual methods. ATS may alsobecome necessary when the load in peak periods approaches 100percent of system capacity.

reduced in number as more supervisory tasks areallocated to machines.

No rail rapid transit system in the United States,or anywhere in the world, is now operating at thislevel of automation. The technology to do this,however, is available; and it has been applied invarious people-mover systems, such as the Morgan-town Personnel Rapid Transit (PRT) and severalairport transportation systems. A notable exampleo f an unmanned a i rpo r t t r an s i t sy s t em i sAIRTRANS at the Dallas-Fort Worth Airport,where small unmanned transit vehicles circulate onfixed guideways over a complex of interconnectingroutes. The entire system is operated and super-vised from a central location by a few persons aidedby a train control computer,

Full Automation

Complete removal of man from control of transitsystem operation-even removing him from thecentral control point—is probably not technicallyfeasible or desirable, For safety and continuity ofoperation, it will always be necessary to have some-one to monitor the system and intervene to restoreoperations or assist passengers in an emergency.The number of such supervisors would be only ahandful, however, and it is doubtful that they couldever conduct normal operations manually as a back-up to automatic systems.

Such a “fully automatic” transit would requirean extremely sophisticated and costly ATC system,which would include ATP, ATO, and ATS for nor-mal modes of operation and—most important—automatic back-ups of these mechanisms for con-tingencies and emergency states, The communica-tion network would also have to be highly sophisti-cated, providing not only voluminous real-time in-terchange among automatic components but alsoextensive two-way voice links between passengersand the supervisory cadre. Another requirement ofsuch a system would be automatic operation,switching, and assembly of trains in yards. Thetechnology for automatic yard operation is availabletoday in rudimentary form in automated freightclassification yards, but it would need to be refinedextensively before application to a rail rapid transitsystem,

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