US2357518A

US2357518A - Switch and signal control system for railroads

Info

Publication number: US2357518A
Application number: US365669A
Authority: US
Inventors: Thomas J Judge
Original assignee: General Railway Signal Co
Current assignee: SPX Technologies Inc
Priority date: 1940-11-14
Filing date: 1940-11-14
Publication date: 1944-09-05
Anticipated expiration: 1961-09-05

Description

Sept. 5, 1944.

T.J. JUDGE SWITCH AND SIGNAL CONTROL SYSTEM FOR RAILROADS Filed'Nov. 14, 1940 15 sheets-sheet 1 T. .L` JUDGE SWITCH AND SIGNAL CONTROL SYSTEM FOR RAILROADS i Sept. 5, 1944.

' F11ed Nov. 14, 1940 15 sheets-sheet 2 ATTORNEY Sept- 5, 1944- T. J. JUDGE .2,357,518

' swITcH AND SIGNAL CONTROL SYSTEM FOR RAILROADS Filed Nov. 14, 1940 15 Sheets-Sheet 3 FIG.. 3.

Control Omte sept. 5, 1944.

T. J. JUDGE SWITCH AND SIGNAL CONTRVOL SYSTEM FOR RAILROADS r Filed Nov'. 14. 1940 15 Sheets-Sheet 4 To f 9.

ToR'NEY i 0:5 0.5169 .COEEOw T. J. JUDGE Y Sept. 5, 1944.

SWITCH AND SIGNAL CONTROL SYSTEM FOR RAILROADS 15 Sheets-Sheet 5 Filed NOV. 14, 1940 INVENTOR d BY ATTQNEY une f Sept. 5, 1944.

T. V.1.v JUDGE SWITCH AND SIGNAL CONTROL SYSTEM FOR 'RAILROADS 15 sheds-sheet e Filed NQV. 14, 1940 T. J. JUDGE 2,357,518

SWITCH AND SIGNAL CONTROL SYSEM FOR RAILROADS Sept. 5, 1944.

Filed Nv. 14, 1940 15 Sheets-Skeet '7 FIGA.

Vlmgy TORQ BY /W ATTORNY Sept... 5, 19A-L T, 1 JUDGE.. 2,357,518 l wITc1-I AND SIGNAL CONTROL SYSTEM FOR RAILROADS Filed Nov. 14, ,1940 15 Sheets-Sheet 8 I 7D I 1 I 211 TF I (4.) 7cpA| I I ai: 6') 7H! GAYGIISDQ@ I I I l Y n |2517 I v al I I I I I .I g -V-z .see Fa. I RCR NCR ATTORNEY 'I I 12r=z I l 4 j IHS* v c+) Picked uP by I FSCTA T. JQ JUDGE Sept. 5, 1944.

SWITCH AO SIGNAL CONTROL SYSTEM FOR RAILROADS Filed Nov. 14, 1940 l5 Sheets-Sheet 9 FIGC.

m m .w/w. W. ea del kme .Me. Pons `nfa |20R N) WHW 3 lNV ATTORNEY Sept. 5, 1944. v T. .1. Ju-DGE 2,357,518

' SWITCH AND SIGNAL CONTROL SYSTEM FOR RAILROADS Filed Novi. 14. 1940 15 sheets-sheet 1o Y I Picked up by C 4WN AYGP Remote Control L (w 4241 T. J. JUDGE A 2,357,518

SWITCH AND SIGNAL CONTROL SYSTEM FOR RAILROADS sept. 5, 1944.

Filed No'v. 14, 1940 15 Sheets-Sheet 11 IOTRA F'lG. .7. E'.

Remote Control Picked up by T. .1.l JUDGEV Sept. 5, 19,44.

SWITCH AND SIGNAL CONTROL 'SYSTEM oR RAILRoADs lNvENo J BY Sept. 5, 1944. T. J. JUDGE 2,357,518

ySWITCH AND SIGNAL CONTROL SYSTEM FOR RAILROADS Filed Nov. 14, 1940 15 sheets-sheet 15 Sept. 5, 1944. T. J. JUDGE 2,357,518

SWITCH AND SIGNAL CONTROL SYSTEM FOR RAILROADS Filed Nov. 14,-1940 15 sheets-sheet-M l Remote Control I 512GB NCR w LTL- auf T. J. JUDGE Sept. 5, 1944;

SWITCH AND SIGNAL CONTROL 'SYSTEM FOR RAILROADS 15 Sheets-Sheet l5 Filed NOV. 14, 1940 FIG.. 8.

RCR

l c-J ININTOR BY h Z 4Z@ ATTORNY Patented Sept. 5, 1944 SWITCH AND SIGNAL CONTROL SYSTEM FORRAILROADS v` Thomas J. judge, Rochester, N. Y., assignor to ,j v

General Railway Signal Company, Rochester,

Application November 14, 1940, serial No. 365,669 v' (c1. e46-3)' t 53 Claims.

This invention relates to switch and signal control systems for railroads, and it more particularly pertains to the remote control of switches and signals when the automatic control of the signals in the field is dependent upon coded track circuits.

This invention contemplaes the remote control from a control oice of switches and signals at the ends of passing sidings on a single track railroad signalled for traii'ic in both directions, the power operation of the track switches being ofcourse dependent upon safety circuits in the field.; and likewise, the clearing and restoration of signals is dependent upon traic conditions in the field. The automatic control of the signals in the eld, the signals between sidings being entirely automatic, is dependentl upon coded track circuits, there being no line wires connecting signal locations except such wires as are used for communicating switch and signal controls from the control ofce to the various field stations at the ends of the sidings. The control apparatus grouped at each end of each siding constitutes a field station, and each eld station is connected by the communication system to the control oice.

Although communication can be maintained by direct wire control between the control ofce and each oi the eld stations, vit is preferable from the standpoint of saving line wires that the communication system be of the coded type such as that disclosed, for example, in the patent to Judge et al., Patent No. 2,082,544, dated June 1, 1937. Such a system is normally at rest and is initiated into a cycle of operation when controls are to be transmitted. A cycle of operation for the transmission of controls comprises a series of selected impulses characteristic of a particular station and certain switch and signal controls for that station.

The automatic control of signals by coded track circuits provides signal indications comparable to the signal indications provided upon passage of a train in the well-known absolutepermissive-block signalling system. Each block extending between adjacent signal locations has a lcoded track circuit in the system provided by the present invention` except the short detector track sections for Athe power track switches, which have track circuits of the steadily energized type. Each of the coded track circuits has at each end apparatus eiTective to transmit and to receive codes'through the rails of that block. 1n other words, there is a code transmitter and a code receiver at each end of each of the coded track circuits. A `transmitter is said to transmit a driven code when the impulses forming the code are applied or repeated from a power driven impulse coder, the length of each impulse and the number of limpulses per minute being predetermined'by the characteristics of the impulse coder. The transmitter is said 'to' transmit' an inverse code when the impulses of the code it transmits are synchronized to tvinto the `'deenergized or oij periods of a driven code transmitted from' the, opposite end of that track circuit. In'accordance with the present invention it yisprovided that 'the clearing of a lsignal is dependent upon a driven code transmitted from the opposite end of the block indicative'of traflic conditions in advance of that signal'. The inverse code is-used for indicating purposes and -forfapproach control. I

An object of the'present invention is to provide traflic directionstorage means for each signal location whereby the directionr of trahie last established is maintained for the transmission of driven codes through the track circuits until a change in direction is called yfor.V Thus,l .for example, if traflic direction has" last been established for west` boundl traffic (to the left) atthe signal location at each'endof a block, a driven code is transmitted "in thev track circuit ,from the left end of that block to the ,west bound signal governing entrancethereto. i o,

Another object of the present invention is rto provide that the direction storage means for theV signal locations at each ofthe Veld stations can be governed from the control Volice. It is fur-i ther provided thatk a change in such direction storage means at a eld station effects Va similar change of ,the direction storage means provided foreach of the automatic signallocations if the direction andthe signal controls designated for` that field station are such as to require the clearing of such automatic signals. More specifically, for exampleif a signal is to be cleared for governing traiiic from a passing siding into a stretch,

tion storage means at that signal location, andk thereby stops the transmission of code east (toY the right)` of that signal locationto cause the,

tumble-down of signal clearing control apparatus'- for each signal location between that eld.

stationandthe next field station' in the -direcresponding direction to be established by the" direction storage means at that next field station in advance of that signal.

Another object of the presentl invention is toA cause the signal clearing code transmitted from the control office during a control signal to a given field station to also be received at the next neld station in advance of that signal, and to be effective at that advance eldstation to cause correspondingly the establishment of the direction storage means.

Another object of the present invention is to delay a change in the direction` of the coding in the track circuits in accordance with approach or time locking under operating conditionsl in which such type of locking are generally involved.

Another object of the present invention is to so organize the system thatan attempt by an operator inadvertently to reverse the traffic direction establishing means fora stretch of track occupied by. a train is-ineiiective tostop the trans- 4mission of track circuit codes` essential-to the clearing (if-signals governingpassage of the train tothe next field station; More specically, for example, the entrance ofy aitrain into a stretch of single track between sidings insures that a vchange in the direction storage means cannot be'effecte'd at the next eld station in advance of the train so as to cause the tumble-downl of the automaticf signals governing passage of that train -to the next field station in advance, irrespective of the number of automatic signals involved.-

Another object of the-present invention is to employ the control codes transmitted from the control oice for the power operation of the track switches to determine whether or not traflic direction is to be established atthe next eld station in advance.' Thatv is', for example,.when a control cycle is transmitted for clearing a signal governing entrance of a train-into a siding, the track switch at that end of the siding is called for to be operated to a reverse position, and such beingthecase itis not necessary to condition the traflic storagemeans at the next eld station inV advance. However, if the train instead of entering the sidingis to proceed on themain track, the calling for the track switch'to be operated to a normal position provides-that the traffic direction storage means for the next-field station in advance is properly'conditioned to provide track circuit-code for clearing the signal governing train movement over that-tracku switch in" its normal position. Such mode of-operation is' desirable, for example, where non-stop meets of trains are involved.

g Other objects, purposes and characteristic features of the present invention will be in part obvious from the accompanying drawings -and in part pointed out as thedescription of the invention progresses.

In describing the invention in detailfreference will be madeA to the accompanying drawingsV which corresponding reference characters are used to designate corresponding parts throughout the various figures, in which like letter reference characters are used to designate parts having similar features and functions, such parts being generally made distinctive by reason of ipreceding numerals or by exponentsrindicative of the particular locations or functions with which such devices are associated, and in which- Fig. 1 shows diagrammatically the general organization of one embodiment of the present invention together with a code chart showing the Icodes assigned to the control from the control oii'ice of various functions at the various eld stations;

Figs. 2A, 2B and 2C show diagrammatically in a simplified manner the effect of a change in direction of traffic and certain traflic conditions upon the system provided for this embodiment of the present invention;

Fig. 3 shows the circuits for the selection of Icontrol codes for transmission from the control oflc'e to the field stations;

Fig. 4 illustrates the control office stepper and various control apparatus and circuits associated with the transmission of control codes to the eld and the reception of indications transmitted Afrom the eld;

Fig. 5 illustrates in detail the stepper and code receiving apparatus and circuits at a particular field-station as typical of apparatus provided at each of the other eld stations in this embodiment of the present invention.

Fig. 6 illustrates as typical for all field stations the decoding means employed irl-connection with the reception of codes by the stepper shown iii Fig. 5 fora'particular field station, and the manner of execution for the energization of application relays more directly employed in the control of switches and signals and traffic direction at that iield station;

Figs. 7A, 7B, 7C, 7D, 7E, 7F, 7G and 7H, when placed side by side illustrate in detail the coded track circuit contro-l of the signals for this embodiment of the present invention; and

Fig. 8 shows certain switch and signal control circuits for one end of a passing siding as typical of theV circuits associated with each end of each ofthe passing sidings in this embodiment of the present invention.

Various parts and circuits constituting this embodimentv of the present invention have been shown diagrammatically for the purpose of simplifying the'illustration and facilitating in the explanation thereof, rather'than for the purpose of showing the speciiic construction and arrangement of parts that would be employed in practice.

The various relays and their contacts are therefore illustrated in a conventional manner and symbols are used to indicate connections to the terminals of'batteries or other sources of electric current instead of showing all of the wiring connections to the terminals. Some relays for convenience have been shown in more than one figure, andsuch relays can readily be identified as being the same by the use of similar reference characters, some relay contacts being shown in the conventional written-circuit manner and identiiied as of a particular relay by the relay reference character above each of such contacts. l The symbols,(|) and are employed to indicate the positive and negative terminals re'- spectively of suitable batteries or other sources of directcurrent, and the circuits with which these symbols are used always have current flowing in vthe same direction. be used the symbols (-1-) and are to be con- If alternating current is to sidered as instantaneous relative polarities.

Y In order to simplify the description of the present invention, reference is made from time to time to functions common to all'parts of a similar character by use of the letter reference characters without exponents or preceding numerals. It is to be understood that such a reference applies to any parts designated in the drawings by reference characters that are similar except for numerals associated therewith.

Track: layout.-With reference to Fig. l, the track layout of this embodiment of the present invention is illustrated as comprising a stretch ,of single track having passing

sidings

20, 2| and 22, only theright end and left end of sidings and 22 respectively being shown.

Each end of each siding has a power operated switch, a detector track section, and signals for governing trame in both directions. For example, at the left end of siding 2| the track switch 3W is power operated by a switch machine (see Fig. 8), such as the switch machine disclosed in the patent to W. K. Howe, Patent No. 1,466,- 903, dated September 4, 1923, and the detector track section is of the well-known steadily energized type with track battery fed in both directions over the track switch 3W. Signal 6A (see Fig. 1) is provided for governing east bound traic (to the right) over rtrack switch 3W in a normal position, and signal 6B is provided for governing east bound trailic over track switch 3W in its reverse position. Signals 'lA and 1B govern west bound traffic (to the left) over 'track switch 3W, signal 'lA governing traic over track switch 3W in a normal position, and signal 1B governing trac over track switch 3W in its reverse position.

- Between sidings 2B and 2| is a double automatic signal

location having signals

4 and 5, signal 4 being provided for governing east bound traiic and signal 5 being provided for governing west bound trafc. Between sidings 2| and 22 are automatic staggered signals I0 and signal l0 being provided for governing east bound tra'ic and signal being provided for governing west bound traiiic.

The signals illustrated are of the search light type, such, for example, as the search light signal disclosed in the patent to O. S. Field, Patent No. 1,835,150, dated December 8, 1931. Each of the signals when deenergized provides ared indication for danger or stop, when energized with one polarity provides a yellow indication for caution, and when energized with the opposite polarity provides a green indication for clear. Although search light signals are shown in this embodiment of the present invention it is believed to be readily apparent to those skilled in the art how other types of signals such as colorlight signals having individual color lamp units, positioned light signals, and semaphore signals can'be employed.

For convenience in illustrating the indications displayed by the various searchlight signals, signal symbols are used having red, yellow, and green indications illustrated by lines comparable tothe position of the arm of a semaphore signal.` Each symbol havingl anv arc (see signal GA, Fig. 7B) is a 'semi-automatic stick signal.

The locked position of each of the track switches in correspondence with the lockedY conditionjof the power switch machine associated therewith is repeated by a polar neutral relay -field stations.

WP inthe usual' manner. 'Such relay is energized with one polarity when the switch points and switch machine are locked in a normal position, and with the opposite polarity when such switch points and switch machine are locked in a reverse position. When the switch machine and/,or switch points become unlocked from either of such conditions, the relay WP-is dropped away. z

Thetrackway is divided into track sections by -insulatedjoints inthe usual manner, there being one track circuit for each block extending between signals.` Offthe track'circuits provided for this embodiment of the present invention, the circuits for :the vtrack sections 5T, 8T, IST and |4T are fed with-steady energy, and the circuits for the. track sections 4T, 6T, 1T, ST,

tery, a track relayl TR or TRA, and the Atrack circuit is impulsed when code is being trans- Initted from that end-of the block byan impulse relay CP or CPA.

It is believed that it will be readily apparent as the description progresses how the system can be applied to practically any track layout of the character of the track layout chosen for this embodiment, each signal location-being considered as a typical unit which can be used in various combinations with other units. For example, the stretch of track betweenV sidings 2| andr 22 could also include a7 double intermediate signal locationhaving/s'ignals such as

signals

4 and 5 if the length and lcondition of such stretch of track 'requires such an allocation of the signals.

Communication system devices-For the purpose of simplifying the disclosure of the present invention, reference characters have been used for communication devices similar to reference characters for similar devices shown and described in detail Iin the above mentioned patent to Judge et al., Patent No. 2,082,544, in order that reference 'can' be made to such vpatent for a more detailed description of the apparatus and inode oi operation of the communication sysem. l

The control oflice communication apparatus comprises change' relays CH (see Fig. 3) used in initiating the Vsystem into operation responsive to the manual designation of controls to be transmitted by the yactuation of switch and signal control levers SML and SGL respectively. and start buttons sB. 'The relays CD, ons, .CHP and C are provided to maintain a properorder in the transmission ofv controls .when controls for more thanl one eld station are designated to be transmitted while the system is in operation. r'Ifh'e selective energization of the relays NC land PC applies the codes of selected polarities tothe line circuit for transmission to the v The relays SA and 2SA (see. Fig. 4) are used ascycle markingrelays and the relays MF, MB, DV, and FCA (see'Fig. 3) are used in connectionwith the reception of indications. The relays Ijl and EP (see Fig. 4) are associated with the impulsing of theline circuit, the line relay F follows vthe impulsing, and its repeater relays FP and 2 FP operate vthe control office stepper consisting4 of steppingn relays V, 2V, 3V, 4V. 5V,'LV and theV half-step relayV VP.

The communication apparatus for each of the eld stations (see Fig. 5)@comprises a line relay F, a line repeater relay cycle marking relays SA and SB, relays PF, PB, LO and CH associated with the transmissiony of indications, decoding relays PS and'NS for eachy impulse included in a control cycle of operation, anda field station stepper consisting of stepping relays lV, 2V, 3V, 4V, 5V and LV together with a half-step relay VP. Relays WN and WR (see Figs. 6 and 8) are provided for each field field4 station for caus ing thc respective normal and reverse poWer Operation of the track switch at that eld station, and relays RGZ and LGZ are provided for causing the clearing of east bound and West bound signals respectively for each field station. Each of the iield stations also has a relay B for the purposev of causing the restoration to stop of a signal in accordance with the transmission from the control cnice of a stop code for that iield station.

In order that the direction of traii'ic may be established in the coded track circuits in accordance with the direction of trailic allowed for by signal controls transmitted from the control office, a code responsive traic direction relay RFZ is provided for east. bound traffic and a code responsive relay LFZ is provided for west bound traffic for each of the iield stations.

Each of the field stations instead of having an SO and SOS relay as shown in the above mentioned Judge et al. patent for selecting only one station to remain in communication with the control oiiice during a control cycle, is allowed to step throughout the cycle, and the selection of the station to receive the controls transmitted is selected by the decoding bank of relays. The stepping of all of the field stations throughout a control cycle is providedfor the purpose of traffic direction control in a manner to be hereinafter described.

If it is desired that the system of elimination of field stations upon the reception of controls as described in the above mentioned Judge et al. patent by the use of SO and SOS relays be employed, it can be provided that instead of all stations being eliminated but one, that all stations will be eliminated but two, namely, the eld station for which the signal controls are intended i and the next field station in advance of such signal. Such an arrangement could be provided by the use of additional SO relays for each station, each controlled in a manner comparable to that shown for the SO relay of the above meni.

tioned Judge et al. patent. Under such conditions one of the additional SO relays would be maintained energized upon the reception of the station code for the next station to the left, and another additional SOrelaywould be'maintained energized upon the reception of the station code for the next station to theright. Each of such additional relays when maintained `energized throughout a control cycle would cause the proper conditioning of the traic direction sto-rage means at that eld station. The use of SO and SOS relays for station selection of course eliminates the necessity of decoding the impulses used for station selection.

Track circuit and signal control devices.-Two track relays are provided foreach coded track section. A track relay TR is provided for the left end of each track section and a-track relay TRA is provided for the right end. The polar structure of the track relays is such that they are picked up only when energized with a certain polarity, thus minimizing the possibility of improper energization from an extraneous source.

Each signal location has two traffic direction relays RF' and-LF (see Fig. 7A), the relay RF being energized when east bound traffic is established and the relay LF being energized when west bound tralic is established. Inasmuch as one of these relays is normally maintained energized in accordance with the last direction established, the relays' RF and LF for each signal location can be Said to constitute a traflic direction storing means.

A track repeater relay TP is provided for each signal location, and such relay repeats the track relay for the track circuit to the left if west bound trai'lic is called for by the tralic storing means, and it repeats the track relay for the track'section to the right if east bound traiiic is established by the traflic establishing means at that location. Such is true of automatic signal locations in the stretch of track between sidings, but the track repeater relay 'IP for the ends of track sections at the ends of the sidings repeat only for one direction of traiiic. At the sidings directional track repeater relays RTP andVLTP are employed, the relays RTP being energized only when traffic direction is established for east bound traic, and the relays LTP being energized only when trailic is established in a west bound direction.

A normally energized approach control relay AR is provided for each signal location, such relay being energized when either a driven or an inverse code is received from either direction at that signal location. Such approach relay is used in providing track occupancy indications and in providing approach control.

A relay H is Iprovided for each of the signal locations for controlling the signals at that location in accordance with the reception of code through the track circuits from the next signal in advance, one H relay being suflicient, even for a double signal location, because of the use of the traic direction relays LF and RF for selecting the signal to be cleared. Each of the H relays is energized responsive to either a code of '75 impulses per minute or a code of 180 impulses per minute. Although specific code rates of and '75 impulses per minute are employed in this embodiment of the present invention it is to be understood that different code rates could be used.

Each of the H relays is energized by a decoding transformer TF, such transformer having its primary winding energized during the on period of the code with one polarity, and being energized with the opposite polarity during the off periods of the code. An alternating current is therefore induced in the secondary winding of the transformer, and such alternating current is rectified so that a direct current relay can be used for each of the H relays by the use of a con tact in the circuit for such H relay of the code following relay TP for that signal location.

A relay D is provided for each signal location for use in selecting a clear or a caution indication to be displayed by the signals at that location, such relay being energized by a tunedv circuit 'to be responsive only to the reception of a track circuit code at a rate of 180 impulses per minute.

A decoding transformer TFA having a primary circuit tuned to the 180 code is pro-vided for each signal location for the energization of the D relay at that location. The primary Winding of each of the transformers TFA is energized by the induced voltage set up upon the collapse of the ux in the primary winding of the transformer TF at that signal location whenever the primary winding of such transformer TF is de.-

energized. Inasmuch as the primary winding of the transformer TF is energized iirst inV one direction and then the other, the primary winding of the transformer TFA for that signal location is energized iirst in one direction and then the other and therefore induces an alternating current in the circuit for the secondary winding of that transformer, such alternating current being rectied so that a standard type direct current relay can be used for each of the D relays.

Code transmitter relays CT and |80 CT are provided for certain of the signal locations for measuring the driven impulses to be applied to the coded track circuits. Such code transmitting relays can be, for example, of the motor driven type, or the oscillating type. The coders illustrated for this embo-diment of the present invention are of the oscillating type having a magnetically rotated balance acting in opposition to a clock spring to provide an oscillating operation. By using balances of diierent weights, the different desired codes are produced.

Two code repeater relays are provided for each coded track section. A code repeater relay CP is provided for impulsing each track circuit at the left end of the track section, and a code repeater relay CPA is provided for impulsing each track circuit at the right end of that track section. The structure of each of the relays CP and CPA is similar to the structure of the relays TR and TRA in that energization of a certain polarity is required to pick up the relay as indicated by arrows in the drawings. Each relay CP or CPA when following the impulses of a code transmitter 15CT or IBUCT, impulses the track circuit to transmit a driven code, and each relay CP or CPA when energized by the secondary winding of Ythe transformer TF for that signal location upon the collapse of flux in such winding impulses the track circuit to transmit an inverse code.

A stick relay S is provided for each of the automatic signal locations for purposes in accordancewith the general principles of absolute-permissive-block signalling systems. Only one S relay is required for each location even if itis a double location because of the use of direction selecting means at that location.

Signal clear and signal stop repeater relays YGP and RGP respectively (see Fig. 8) are provided in the usual manner for repeating the condition ofthe various search light signals.

Normal and reverse correspondence relays NCR and RCR respectively are provided for each track switch for repeating the correspondence of the track switch with the position of such track switch last called for by switch controls received Operation Ge1zeral.-Theswitch and signal control system provided by this invention allows traffic to be estiblished in only one direction at a time over a stretch of track extending between sidings.

Such arrangement prevents the simultaneous entry of opposing trains into that stretch of track on clear or caution signals, as-the signals for but one direction can be cleared at a time Thus, with reference to Fig. 1, for example, if trafiic is established for west bound trains, the signals for west bound traffic can be cleared for the stretch of track extending between passing sidings 2| and'22, but the signals for east bound tramo over that stretch of track must remain at stop.

It is further provided that approach locking is effective to prevent a change in direction when a train is about to'enter a stretch of track between sidings, as, for example, when a train is about to enter the stretch of track extending between sidings 2l and 22 upon accepting signal v| 3A (it being assumednsuch signal to be cleared by an operator at the control office) If the track'section in advance of signal 13A has been entered by the train, the clearing of an opposing signal is prevented according to the principles of automatic permissive block signalling systems, and if the train comes toa stop after an operator attempts to change the direction' of traic by rst restoring the signal I3A to Ystop without passing that signal the reversal in direction of traic can safely become eiec'tive at the end of the time interval provided bythe approach locking.

The coded track circuit system provided in this embodiment of the present invention is of the normally energized type. That is, code is normally applied to the track for one direction of traflic or the other, the4 direction last established being maintained until 'the opposite direction is called for. Thus, lit is provided that if a signal control is transmitted from the control oilice for clearingv a signal for the direction of tra'c already established by the traic' direction storage means, the signal is cleared as soon as the controls are received because a driven code is already being fed through the rails from the opposite end of the block in advance of that signal. However, if a Signal control is transmitted from the control oiice for a direction opposite to that already established by the traffic direction storage means, the reception of such control stops the transmission at that signal location of the driven code for the opposite direction providing an kinverse code is being received, and conditions the entering end of the block governed by that signal to receive a driven code transmitted fromthe opposite end of the block. At the same time, the signal clearing code is also received at the next field station in advance of that signal, and the reception of Vsuch code at that signal location causes the picking up of a traic direction relay to correspond with thedirection to be established. The picking up of that relay. conditions traic at that location in accordancewith the direction of traiiic called for in order to' clear the desired signal so as to cause the transmission of a driven code back toward thatsignal.

It is therefore provided that each signal clearing code transmitted from the control oice is