Good news for commuters:better and faster rapid transitA comprehensive discussion of rail-transit-control concepts-from signal systems to automated train operation-for increasingpassenger capacity, providing more reliable service and more'rapid' rapid transit for the urban and suburban rail traveler

Mayer Horn Tri-State Regional Planning Commission

Rail signal systems have traditionally been employed "safe-braking distance."to prevent collision of trains while permitting them to Basic to rail signal systems is the subdivision of trackadhere to a schedule that is adequate for moving pas- into sections called "blocks," in which trains are locatedsengers and/or freight. In today's rapid-transit sys- (see Box A).tems, however, the corollary objectives of a signal Information concerning the location of a given train issystem-namely, controlling the speed of trains, in- transmitted to following trains by means of waysidecreasing the capacity of the line, and minimizing signals, which are spaced along the track to indicateschedule disruptions caused by transient disturbances whether the blocks ahead are occupied. But the engine-in train operations-are critical to providing satis- man or motorman must actually see the wayside signalfactory service to passengers. before he is aware of the condition in the block(s)

ahead. (More on wayside signals in Box B.)A better system transmits signals to the train's cab so

that the operator can adjust to conditions ahead before heNumerous cities in North America, and around the actually has the wayside signal in sight. (For more de-

world, are either building new rapid-transit systems or are tails on cab-signal systems, or CSS, see Box C.) Fi-modernizing and extending existing lines. How does nally, the transmission of track conditions by CSS becomesthe performance of these new systems differ from the the basis for automatic train operation (ATO), which willolder, conventional ones? be discussed later in this piece.At present, the minimum time between trains-called

"headway"-ranges from 2 minutes on urban systems to The 'rail-transit expressway'3 minutes on suburban lines. This rate of 20 to 30 trains Trains in an ideal system (one without main-line stops)per hour has not changed since the turn of the century; could have two to three times the capacity of a conven-neither has the concept of stopping all trains at "main- tional system whose capacity is limited by station stops.line" stations instead of at sidings. This practice not only There is a parallel-well known to transportation engi-holds passenger capacity down, but also restricts the neers-that the capacity of a main-line lane on an auto-average speed of trains. motive expressway is about three times the capacity of aOf the three techniques to increase line capacity-more lane on the same road at a toll barrier. The transposition

tracks per line, more cars per train, and more trains per of this concept to rail transit can be viewed as the "rail-hour on each track-the third technique seems to be transit expressway."overlooked too often. But this technique can be achieved Applications of the rail-transit expressway concept.by improved train control. In two new systems-BART In our largest conurbations (New York, Tokyo, London,and the Washington (D.C.) Metro-train control will etc.), service between outlying areas and the centralinclude computers to optimize train operations by per- city-especially on costly sections such as river tunnels-forming the following tasks: can be provided on rail-transit expressways where effec-

. Adjusting station dwell times for schedule control. tive advantage can be taken of the capacity and speed

. Assigning numbers to each train as it enters revenue of rail-transit lines without station stops. The schemeservice. is also applicable to intercity railways.

* Monitoring electrical and mechanical support Design criterions and constraints. In applying this newsystems such as ventilators and power substations. concept, and in determining the capacity of specific lines,

* Limiting acceleration of trains for both schedule the design criterions and constraints of the train-controlcontrol and current reduction during power substation system are of primary importance. (These criterions andoutages. constraints are also valid for conventional systems.)

Train control, today, consists primarily of a signal One of the basic criterions of signal system configura-system that is employed to permit trains on a line to tions, as well as their components, is that they must bemove at the highest possible speed (consistent with curves, of fail-safe design. Redundancy is generally unac-track switches, etc.) and minimum acceptable distance ceptable in meeting this criterion; each of the systemsfrom the train ahead. In railroad jargon, this is called must be fail-safe.

IEEE spectrulm FEBRUARY 1972 55

formance of the follower, but also on the location of theleader-without considering its speed. This is true even

A. Block signal if the leader's speed is measured in a fail-safe mannersystems and train detection and is continuously transmitted to the control system

and/or the follower in a fail-safe manner. This require-Conventional signal systems employ techniques ment is based on the possibility of a leading train de-

in which a track is subdivided into a series of railing and stopping almost instantaneously-for ex-sections called "blocks." Trains are detected as ample, by striking a concrete abutment. The spacingbeing in one or more blocks. The last block oc- of a following train, at least at safe-braking distance, iscupied by a train (i.e., the block occupied by the intended to prevent the compounding of the disaster.rear axle of the last car of the train) causes the But there is an even greater danger of a train derailingfollowing blocks to transmit various signals. into the path of a train moving in the opposite directionOperations are based on the principle that a on an adjacent track. Although the consequences of theblock occupied by a train causes the following resulting head-on collision would be at least as seriousblock or blocks to permit speed limits that as those produced by a rear-end collision, this possibilityensure that the following train is, at all times, a is almost never considered.safe-braking distance from the last block occupiedby the leading train. Block lengths vary from a Automatic train operationminimum of about 35 meters on some urban Automatic train operation involves the use of car-transit systems to a maximum of about 1600 borne equipment (not necessarily fail-safe) that willmeters on main-line railroads. apply power or braking to govern the actual speed toMany innovative schemes for train detection within a small tolerance of the cab-signal indication.

have been rejected for fail-safe deficiencies. Only Thus ATO can be compared with manual train operation,continuous-track circuits for train detection are using cab signals and overspeed protection, by listinggenerally acceptable. These circuits may be either their similarities and differences:ac or dc, depending on the type of power available 1. Both ATO and manual operation with cab signalsfor train propulsion. require the same wayside equipment.One modification to the block signal system is 2. Both utilize fail-safe systems to assume control

the use of a pair of parallel conductors (for com- upon detection of overspeed. (Overspeed can be definedmunications, train detection, and train control) as exceeding a tolerance above the command-speed limitinstead of the track circuit. A primary advantage or, when a train crosses a block boundary and receivesof this scheme is that lightweight cars are as easily a lower-speed command, failure to apply the brakes atdetected as regular trains. an appropriate rate.)

3. Both employ nonfail-safe systems for normal trainoperation.

4. A policy decision is generally made by managementsof automated train systems to retain a train "atten-dant."* (Typically, in the event of ATO equipment failure

All systems in train control are divided into two cate- the attendant can operate the train in a conventionalgories: vital and nonvital (depending upon whether the manual mode, utilizing the cab signals.)consequences of a failure affect life and limb). All vital A block design (including block lengths and speedsystems must be fail-safe. For vital systems, it is in- commands in each as a function of occupancy of blocksvariably assumed that ahead) can be the same for either an ATO system or

1. All relevant nonvital systems will fail at exactly manual operation with cab signals, unless different reac-the most critical instant (with a probability of unity). tion times or braking rates are permissible in response

2. Either all vital systems will never fail, or considera- to a reduced-speed command.tion of such failure is beyond the scope of the designapplication of these systems. Automating other on-train functions

Actually, no system is completely fail-safe in reference With ATO, the motorman does not necessarily haveto all types of accidents (or sabotage), or in regard to nothing to do; many functions remain, which can eitherall possible times of occurrence of these failures. For be automated or handled in a more conventional manner.example, the failure of a rail joint can go undetected- These include...even with track circuits-if the bonds remain intact. Station stopping. Automatic station stopping isOne of the requirements of the fail-safe design con- generally termed "program stopping." This does not

cept is that the signal system enforce train separation of imply any software program; rather, it indicates eitherat least safe-braking distance. This train separation, several checkpoints are used to point out "distance tofor a pair of successive trains operating in the same di- go" to center the train properly along the passengerrection on the same track, is the distance from the rear platform, or there is a continuously calibrated measure.Of the "leader" to the front of the "follower." Safe- Without program stop, the motorman must assume con-braking distance, then, is also the distance required for trol from the ATO equipment to effect station stopping.

the olloer ostp sfelyundr th wost st o assmp- * Westinghouse Electric Corp. has been testing and marketing-tions: the longest possible human and equipment reac- notably in the Pittsburgh area-its unattended hybrid vehicletion times, and the smallest possible deceleration rate. (synthesizing features of rail transit, bus, and elevator), variouslyThe concept of safe-braking distance, however, is termed "Skybus" and "Transilt Epessway."n SeveralSkybusac

based not only on the worst possible stopping per- Skybus is on its own track, control is similar to elevator control.

56 IEEE spectrum FEBRUARY 1972

B. Wayside signal systems or "key-by.") There is no enforced compliancewith any less restrictive indication.

The transmission of information to trains, and The bottom left diagram shows the indications ofmeasuring and controlling their responses to this the signal aspects* following occupied blocks ininformation, can be accomplished by the use of two typical three-aspect systems, one of whichsignals located periodically along the track or right (A) applies to rapid transit; the other (B) is ap-of way (wayside). Wayside signal systems, in plicable to conventional railroads. In these sys-which trains are operated manually in all cases, tems, each block has a length of at least safe-are used extensively for both rapid-transit and braking distance and there are three aspects:railroad operations. Train control is enforced in green, indicating that the next signal does notonly one case: Compliance with the most restric- indicate "stop," and that the train operator maytive (stop) signal indication on rapid-transit sys- proceed at the authorized speed; amber or yellow,tems (and some railroads) is ensured by me- meaning that the next signal does indicate stop;chanical "trippers" on the front of the train being and red, indicating stop.engaged by trip stops which are located adjacent The bottom right diagram in this box showsto one of the running rails at the wayside signal. indications of the aspects following occupiedThe engagement of the car-borne tripper by the blocks in two typical five-aspect systems; sketchwayside trip stop automatically initiates emergency- "A" is for rapid transit, and "B" is for railroads.brake application. (An exception, in some sys-tems, allows the train operator to take overrideaction and to proceed beyond some types of stop- * "Aspect" rcfers to what thesalisFor "indica-tion" refers to what the signial mieants. For examnple, "green"signal aspects. This is termed "stop and proceed," is an aspect for which the indicatiotn is "proceed."

A-THREE-ASPECT WAYSIDE SIGNAL systems as A-DIAGRAM OF FIVE-ASPECT wayside signal sys-used on urban rapid-transit lines. B-Three-aspect tem as installed on urban rapid-transit lines.wayside signal configurations as installed on con- B-The same configuration on conventional rail-ventional railroads. roads.

Train Direction of travel - Train AA I I I il l l l I l l

G A R R Direction of travel G/G G/G GIG G/A A/A A/R R/R RIR R/R R/RRapid-transit systems Train Rapid-transit systems Train

B I I I I _I I I I I I I I I IG G A R G/G G/G G/G GIG G/G G/G G/A A/A A/R R/R

Conventional railroads Key: G-green Railroads Key: G/G-Green over green R/R-Red over redA-amber or yellow G/A-Green over amber A/R-Amber over redR-red A/A-Amber over amber

An example of a rail system with ATO, and without Automating station functionsprogram stopping, is the Long Island Railroad's new There are several functions, which can also be auto-M-1 (Metropolitan) cars. By comparison, the Port mated, that do not generally involve train operators.Authority Transit Corporation (PATco) South Jersey Since these are occasionally associated with automatedline was the first in the U.S. to have ATO, and does have transit, it is worth identifying them ...program stopping. The LIRR M-l cars were designed Automatic fare collection. Although automatic faresubsequently (by the same firm) without program stop. collection, using coin-operated turnstiles, has long been aDoor open/door close. In conventional, manually feature of rail-transit systems that have a flat-fare struc-

operated systems, an on-board trainman is responsible ture, graduated fares have traditionally required on-for opening and closing doors at stations. These functions board trainmen, or personnel at each station to checkcan readily be automated; door opening can be initiated passengers as they entered and left the transit system.upon satisfying predetermined requirements simulta- Recent developments, involving the use of magneticallyneously-zero motion, with brakes applied; and, the encoded cards, have enabled the implementation of auto-entire train stopped within station confines. Door closing matic fare-collection systems on the suburban Illinoiscan occur upon the expiration of a dwell timer, with or Central system and the PATCO rapid-transit line.without modification by a clock timer. The clock timer, Automatic station monitoring. There is certainlywhich represents an effort to achieve schedule control, nothing new about unattended stations with train-bornecan be adjustable from a central location. fare collection. With station fare collection (coin op-Motion initiation. In an automated rail-transit sys- erated, or automatic collection of graduated fares), a

tem, motion at a station can be initiated upon the closure system employee must be available to give change or sellof the train doors, or by the additional requirement that tickets, to provide information and a feeling of securitya separate Go button be pressed. for passengers, and to respond to unusual occurrences-

Horn-Good news for commuters: better and faster rapid transit 57

C. Cab-signal systems (CSS) Direction of travelI I I ~~~~~~~~~Train

Perhaps the best way to describe CSS, and to 120km/h Okmahidistinguish them from wayside signal systems, is (full stop)to list their differences: DIAGRAM SHOWING THE INDICATIONS following

* Track circuits, used for train detection in an occupied block in a simple, two-aspect situa-wayside systems, are also employed to transmit tion. One of the aspects is "0 km/h" (full stop),signal indications to the train operator's cab. and the other is the permissible speed limit.

* The train operator has continuous indicationof the signal aspect with CSS. (Thus, when anaspect becomes less restrictive while a train is in a SCHEMATIC DIAGRAM showing the indications

following an occupied block in a five-aspect cab-block, the operator of a train equipped with CSS signal system (CSS).can take immediate advantage of this fact; whereas, Direction of travel------with wayside systems, the operator will not become I I Taware of a signal change until he can actuallj see L k201m2/l2hl20,l20ll2,l20 8l 0 56 35l O -

the next wayside signal.) Safe-braking 5kSBD* Vehicle operation can be either manual or distance (SBD) 1 56km/h

automated with CSS; a system using wayside 80km/h-signals cannot be automated. Safe brakin distance from

* With CSS, control over vehicle speed is 120km/htoOkm/h

continuous; but, control with wayside signalsoccurs only when a train passes a raised trip stop.(Thus, with CSS, the signal engineer can easily * With CSS, if the train speed is above theenforce various speed criterions.) reduced-speed limit, the operator must initiate at

* Lack of continuous control with wayside least a specified braking rate within a prescribedsignal systems imposes a harsh penalty for over- time limit to prevent the automatic application ofspeed and makes signal compliance more diflicult. brakes.(For example, the operator of a rapid-transit train A graphic comparison of wayside signals andwho is approaching a restrictive wayside signal too CSS may be helpful: the top diagram shows therapidly to stop-even though he has full service indications following an occupied block in a simplebrakes applied-will be "tripped" into an emer- two-aspect situation. One of the aspects is for agency-brake application; but, with CSS, the full stop, and the other is for the permissible speedoperator can take action in a similar situation to limit (120 km/h). A comparison ofthisdiagram withprevent such drastic action. the bottom left diagram in Box B reveals that rapid-

* Further control can be achieved by requiring transit systems, with CSS, are not penalized inthe train operator, equipped with CSS, to ac- relation to railroads; also a two-aspect CSS isknowledge certain signals. (Generally, upon cross- equivalent to a three-aspect wayside system.ing a block boundary and receiving a reduced-speed The schematic diagram above shows the indica-command, the operator must acknowledge the tions following an occupied block in a five-aspectreduced-speed limit-even if the speed of the situation. The reader may compare this with thetrain is below both speed limits.) second diagram in Box B.

particularly, a breakdown of fare-collection equipment. the commands to each train to the performance of otherThe PATCO line has combined television surveillance trains. This is a new level of sophistication which is very

with a telephone line between the central-control office exciting to the writer.and the station-control area. The achieved objective The Washington Metro system.t The Washingtonwas to eliminate personnel at each station as part of an Metro system will have a control computer and a backupoverall effort to keep operation costs to a minimum, computer. The control computer will receive and trans-without degrading service. mit messages between the central office and field locations

Computers and on-line control * A coimparisonl call be made of mantning philosophies of auto-Computers ~~~~~~~~~~~matedsystems: the P'ATCO) line emiphasizes the absence of

Beyond automatic operation. Unattended trains and station personnel, but has an attendant on cach train. Westing-stations are technically feasible*; this includes automatic house's Skybus scheme emphasizes the absence of train personiel,automatc

.but says nothing about station manning. By contrast, both BAnTstation stopping, automatic fare collection, and auto- and the Washington Metro, which are heing designed as auto-matic station monitoring. But what makes systems like mated systems, will havc both train and station attendants.

BARr and the Washington Metro-both of which are t Although the writer headed the Operations Analysis Group onunder construction-pioneers in control? The answer the design of the automilatic trainl-conltrol and communicationslies in hierarchical control structures, with centralized systems for the Washinigtot Metro, he has n5ot been associatedwith this project since September 1970. Subsequent changes arecomputers optimizing overall operations, and relating not reflected in the discussion.

IEEE spectrum FEBRUARY 1972

via a data-transmission system. It will also communicate system. Note that only in the event that both computerswith the central-control supervisor to provide CRT dis- fail simultaneously will the Washington Metro systemplays and accept console inputs. Also, the control com- revert to the same state as today's most modern system,puter will perform data logging and-the key item from the PATCO line.an operations viewpoint-minor schedule regulation.The backup computer will perform two functions: Interrelation of signal

it will act as complete backup for the control computer, systems, headway, and capacityand it will modify schedules. The relation between speed and minimum distance

Disturbances or failures can happen to the control separation (rear of leader to front of follower) specifiessystem as well as to the train system. Hence, "normal" the capacity capability of any transport system with givenand "abnormal" apply independently to each. The follow- vehicle lengths. On rail-transit systems, it is the signaling situations provide examples... system that specifies the speed versus distance separationNormal train system with normal or partially failed relation.

control system. When the train system is functioning Minimum headway is thus a function of both trainnormally, the control computer performs schedule reg- length and distance separation. Therefore, headway canulation. This consists of adjusting station dwell times and be expressed as the sum of two termsinterstation run times. Dwell times range between the train length distance separationminimum, which is that required for the estimated num- Headway = speed speedber of passenger interchanges between train and plat-form (with a reasonable margin), and the maximum dwell, For a given train length, the first term decreases hyper-which is the greatest value permitted (limited by the bolically with speed, while the second term increasesdesire not to reduce line capacity and by the threshold linearly whereas distance separation is not constant but,of passenger impatience). rather, proportional to the square of speed. Hence, one

Interstation run times are varied by choosing one of may deduce that minimum headway exists for a rail-four top speeds and one of two maximum acceleration transit-expressway line with specified characteristics-rates. For each station pair, the four top speeds can be and, indeed, this is so. It occurs at the speed equal to theselected from among 12 possibilities. square root of the product of twice the braking rate mul-

Since schedule regulation is performed only in one tiplied by the train length. The minimum headway is thecomputer, failure of either of the computers does not square root of the quotient obtained by dividing twiceaffect the functioning of the train system as long as this the train length by the braking rate. (Any human- orsystem is in a normal state. equipment-reaction time must simply be added to thisAbnormal train system with nornmal control system. headway.) Also, minimum headway occurs at the speed

In the event of disturbances to the train system, which at which distance separation equals train length, if thereare beyond the ability of schedule regulation to correct, is no reaction time; otherwise, it typically increases bythe backup computer will request a stored corrective about 5 km/h. If distance separation is based on a variablestrategy and use this strategy as the basis for modifying braking rate, or includes restrictions on the buildup tothe schedule. The strategy-selection process is entirely full-braking rates (called "jerk-rate control"), there willstraightforward and deterministic. It depends only on the be further-but minor-distortions to these relations.current values of a set of parameters such as location of Figure 1 indicates the form of the relation betweenfailure and time of day (peak or off-peak hours). minimum headway and speed on a rail-transit expresswayTo develop an operational strategy-selection system, when successive trains are operating at the same com-

it is necessary to determine the relevant parameters mand speed, and block lengths are designed for thisthat constitute the decision matrix, to identify the speed. Each curve is the sum of the train-length hyper-various combinations of values for all of the parameters bola (train length divided by speed), plus the linearin the matrix which warrant different actions, and then train-separation distance (speed divided by twice theto describe these actions in a sufficiently specific way to braking rate).enable implementation of schedule modification. Ex- The implications of these relationships are thattensive system simulation is needed to develop a viable * There is an ideal capacity for each rail-transit ex-strategy-selection system. Actual operating experience pressway.can be expected to produce further changes in the * Calculations, using the characteristics of modernstrategy-selection/schedule-modification systems. rail-transit systems with CSS, yield a capacity speed of 50Abnormal train system with partial failure of control or 55 km/h. (This is about the same as the capacity

system. When either computer has failed, there is no speed-more commonly termed the critical speed-strategy-selection or schedule-modification ability. This for motor-vehicle expressways.)is no problem as long as the train system is normal. * Application of the characteristics of a typical urbanSerious abnormalities in the train system necessitate rail-transit system yields a headway of less than 30 sec-intervention by the human central-control supervisor. onds, or about 120 trains per hour.

Complete failure of the control system. In the event To understand better the upper limit of rail-line ca-that both computers fail simultaneously, trains would pacity, this composite example is useful: assume trainstop for just the minimum dwell at most stations, and lengths of 250 meters, 300 passengers per 18-meter-longwould be detained a few additional seconds (until their car in a "crush"' condition (which is certainly not a designscheduled departure times) at those stations that can be condition for comfort, and is not likely to be sustainedtermed "control stations." Of course, train operation for a full hour). The calculation [300 X (250/18) X 120]woulld be fully automatic, as would fare collection and indicates a capacity of more than 500 000 people perall other functions that do not rely on the computer hour per track for arail-transit expressway!

Hlorn-Good news for commuters: better and faster rapidl tralnsit 59

70 ~~~~~~~~~~~~~~thereis need for some positive slack-headway time.

(The duration of the peak-flow rate is one of the pa-° operameters that affects the required sack-headway time.

If the duration is increased, additional slack-headways time is needed to maintain a given level of stability.transi expresswaywhensuccessiveg The second parameter that affects the required slack-

headway time is the variability of operations; this issigned influenced by the control mode of both individual trains

and the train system. Although the writer is not aware20eof any material on which to base quantitative compari-

same train length and human and equipment reacti sons, the following qualitative statements are offered:t A wayside signal system (see Box B), with trippers,* ATOrequires more slack-headway time than does a CSS.

0 20 3 0 S 0 7 0~(The CSS eliminates any uncertainty as to when the train0 Speed (mi/h) operator will observe the next wayside signal.)

i Train operators, with wayside signals, will be moreFIGURE 1. The curves indicate the form of the relation- cautious-because of the consequences of overspeed-ship between minimum headway and speed on a rail- thtransit expressway when successive trains are operating an will operators with CSS.at the same command speed, and block lengths are de- With wayside signals, the significant parameter issigned for this speed. Each curve is the sum of the train- indicated as the distance to bring the train to a stop;length hyperbola (train length divided by speed), plus the this is only partially under the motorman's control.linear train-separation distance (speed divided by twicea CSSato signifin t parameterbances.the braking rate). These two typical curves use the ComWith conthe ignifn paameter ith tentosame train length and human and equipment reaction apply brakes; this is fully under the train operator'stimes, but different braking rates and block lengths. control.

a ATO requires less slack-headway time than doesmanual operation-even with CSS-since the vagaries otindividual train operators are removed.e A train system that is more subject to random de-

The rail-transit expressway concept should be consi- lays requires more slack-headway time. (Such delays in-dered only where there are long runs between stations, elude station dwell times.)In conventional systems, where passenger capacities are Slack-headway time should be distinguished fromlimited by station capacities, blocks between stations slack-running time; the latter refers to the difference be-can be substantially longer than those in and near sta- tween the scheduled running time and the running timetions. Block design depends on safe-braking distance, of some reference train. Some slack-running time isFactors that tend to increase safe-braking distance at a necessary to absorb minor system disturbances.given speed (reduced braking rates and increased response Computer control improves stability. With central-time to call for braking) also tend to increase the fre- computer control being installed at BART and the Wash-quency of incidents in block design when the safe-braking ington Metro, and with the experience that should resultdistance of the follower encroaches on the last block from the operation of these systems, it may becomeoccupied by the leader. possible mathematically to relate slack-headway time to

There are two possible courses of action in this event: slack-running time, to define possible tradeoffs, and toeither relax the headway criterion or locate the leader develop expressions of train-system stability. At present,more precisely by subdividing blocks. But the cost of the all that seems clear is that slack-running times should bewayside portions of train-control systems is essentially recognized in the signal-block layout, that how slack-proportional to the number of blocks; thus, subdividing running time is normally consumed can affect line capac-blocks quickly becomes very costly. Further, this ap- ity, and that excessive slack-running time can decreaseproach is characterized by diminishing returns as train capacity.separation equals safe-braking distance (rounded up to A synthesis of the diagrams in Box C and Fig. 1,an integral number of blocks), plus one additional block, indicates three important conclusions:

In summary, the signal system presents a cost-effective 1. Reducing average block length increases capacity.opportunity to increase capacity (and quality) by in- 2. If additional signal commands are introduced,creasing acceleration-deceleration rates, eliminating un- there is no further increase in capacity at top speed.necessary reaction times, and using CSS-preferably 3. The introduction of additional commands does in-with ATO. crease the capacity at lower speed limits. (This capacity

increase can be designed to provide a form of trafficStability and practical capacity regulation; it also can be used to increase system stability

Practical, or operable, capacity is based on the smallest significantly.)possible headway that can be maintained for some givenperiod, and the schedule is then based upon this headway. Future developmentsThe elapsed time, after a signal clears, uintil the next Line capacity. The line capacity on current rail-transit

train is schedued to arrive can be termed the slack- systems, unlike any other mode of transportation, is re-headway time. With zero slack-headway time, the stricted to the capacity at stations. To increase the capac-system is inherently unstable. Any distuirbance to the ity of present line-haul sections (and to make capacitysystem of trains will cause a delay which cannot be cor- more nearly proportional to higher speeds, instead ofrected-at least-until the supply rate drops. Hence, peaking at 50-55 km/h), additional developments would

60 IEEE spectrum FEBRUARY 1972

be required. For example, present signal-system design can foresee a system that ranges from demand re-does not take advantage of the speed of the leading train sponsive during off-peak periods to fixed interval duringin determining which signal aspect to display or trans- peaks; but, at all times, no intermediate stops would bemit to the following train. To use this information in made.operating trains within safe-breaking distance, there are The comparison of schedule control for bus and railthree main avenues of attack: systems is interesting. With centralized traffic-control

1. Determination of the parameters that affect the de- (CTC) systems (increasingly common on rail lines), theceleration rate of a train as it is being derailed, and use occupancy of blocks is visually displayed to dispatchers;of these inputs to design a system that is fail-safe in re- also, two-way radios are almost standard equipment to-gard to rear-end collisions. day. Thus, in rail ystems, all information typically re-

2. Determination of the conditions necessary to make quired for decisions is provided, and the desired level ofa derailment impossible. (Some of the tracked air-cush- control is not inhibited. By contrast, two-way radios onion-vehicle systems under study may achieve this.) buses are more the exception than the rule, and R&D on

3. Application of the National Transportation Safety bus location and identification is just getting under way.Board's "system-safety engineering principles" to the en- For example, a bus-location system is being developed intire train-control system.' (For example, which poses a Chicago, and the writer is participating in the develop-greater threat to a train in the event it derails: its follower, ment of systems for all vehicles on streets and highways.or a train operating in the opposite direction on an ad- Clearly, rail-system control has a headstart relative tojacent track?) controls on bus systems.

Perhaps the fail-safe concept has outlived its useful-ness, and the applications of probability theory to the REFERENCEvarious consequences would produce systems which are 1. A Study of Washington Metropolitan Area Transit Authority'smore cost-effective. Safetjy procedures for the Proposed Metro System. Washington:

Thc National Transportation Safety Board, Sept. 1970.New switches and braking systems? If trains, which are

less than safe-braking distance apart, are to be merged RECOMMENDED READINGand diverged at junctions, conventional fail-safe prin- Berry, D. S., et at., The Technology of Urban Transportation.ciples require the development of switches that cannot Evanston: Northwestern University Press, 1963, pp. 43-49.fail in an open position.* The writer is not aware of any Capacities and Limitations of Urban Transport Modes. Washington:"infallible" switches that have been developed or are in Institute of Traffic Engineers, May 1965.the R&D stage. On the other hand, proposals to accom- Highway Capacity Manual. Washington: Highway Research

Board. 1965.plish all switching aboard rail-transit vehicles (with only Traffic Engineering Handbook. Washington: Institute of Trafficpassive wayside elements) would eliminate the need for Engineers, 1965.infallible switches.With very close headways, and with the speed of one

train determined by the speed and location of the pre- Reprints of this article (No. X72-026) are available tovious train, it would be necessary to have accurate knowl- readers. Please use the order form on page 8, whichedge of the variability of braking characteristics of trains gives information and prices.in a given system. If this variability is unacceptable, theneither increased maintenance or the development ofdifferent types of braking systems may be required. Maofyorridornidmonstera

Control systems. Perhaps the areas in which the most oionso forr demonstra-consistent improvements will be made is that of the de- Regional Planning Com-velopment of controls to optimize system operations in l mission (the transporta-response to normal and severe disturbances. Instead of tion and regional plan-attempting to optimize the actual operations against a ning agency for the Newfixedschedule,toPhizeschedule itself will be continually i 111! g York metropolitan area).fixed schdule, th schedul itself ifl be cntinuaUyHe is the project engi-under modification, based not only on the changes neer on a federallyin the transport system supply but also in response to funded project to improvechanges in rider demand. commutation through theThe concept of varying scheduled times and routes is most heavily traveled bus

corrdorin he .S.Mr.Horn was previously withnow being applied to minibus systems under several a consulting engineering firm which was part of anames, notably "dial-a-ride."t Why not rail transit? Why joint venture to develop the Long Island Railroad'sshould persons entering a rail system (whether urban modernization program. Here, he subsequentlytransit or suburban commuter) at one station, and des- headed the Operations Analysis Group on thetined for another, be compelled to stop at numerous in- tions systems for the Washington, D.C. Metro.termediate stations? This is preposterously obsolete! One He was also associated with the New York City

Transit Authority.* In recognizing the relation between speed and headway, one can Mr. Horn received the bachelor's degree in civilenvision a system of moderate-speed mergings and divergings at engineering from The Cooper Union in 1962, thejunctions in which trains are separated by safe-braking distance; master's in the same discipline from M.I.T. in 1964,on the express section, these trains could be operating at higher and the master's degree in business administrationspeeds, separated by less than safe-braking distance. in 1968, from the City College of the City University of

t In this system, a subscriber establishes credit with a transporta- New York. He is a licensed professional engineer intion service. He may then use his telephone to dial encoded New York State, and holds memberships in thenumbers to obtain bus service-literally from his "front door"- ASCE, the Institute of Traffic Engineers, and theat the time when he needs it. and to go directly to his destination American Railway Engineering Association.either nonstop, or with a minimum number of intermediate stops.

Horn-Good news for commuters: better and faster rapid transit 61