US2134952A - Automatic train control system - Google Patents

Description

Nov. 1, 1938.

W. T. POWELL `AUTOMATIC TRAIN CONTROL SYSTEM Filed Feb. `18, 193'74 2 Sheets-Sheet l v mL etang .zmuu LEEF bdxud. A wm IIET EMF m, MIIU W www L 2HE.

Nov. 1, .1938. w. T. POWELL 2,134,952

AUTOMATIC TRAIN CONTROL SYSTEM Filed Feb. 18, 1937 2 sneetssheet 2' lNVENTOR "T ATTORNEY Patented Nov. 1, 1938 2,134,952 l AUTOMATIC TRAIN CONTROL SYSTEM Winfred T. Powell, Brighton, N. Y., assignor to General Railway Signal Company, Rochester,

Application February 18, 1937, Serial No. 126,423 18 claims. (Cl. zie-63)Y l This invention relates to automatic train control systems for-railroads and more particularly to an intermittent inductive system of the inert track element type, in which means are employed for transmitting signal indications between the wayside and the train.

g The system of the present invention is characterized by the `use ofV an inert wayside circuit', by the transmission of energizingelectric energy from the train to the wayside circuitfthe transmission Of the energy back again tothe train after having been controlled in accordance with traffic conditions and the actuationof the signal devices on the train by the return transmission. The general purpose or object of the present invention is to provide asafe and efficient means for discriminating between the outgoing and the return transmission of electrical energy and of thereby insuring that the energy which is effective in operating the signal device is derived frorn'theV outgoing energy and that it has been subjected to wayside controlin accordance with traffic conditions, e

More specically, distinctive clear,lcaution and stop indications are transmitted from the wayside to the train by the intermittent inductive cooperation of receiver carried inductors `on a railway vehicle and inductors located along the track, whichinductors comprise simple magnetic cores and coils energized only from a source of alternating current on the vehicle,.and which vprovide an organization in which the receiver on the vehicle will not be adversely iniiuenced by extraneous magnetic fields along the trackway and in which transmission of the indications is effective, without changing circuit connections or the like, for movement of the vehicle in either direction past the inductor location or with either end of the Vehicle ahead.

The present invention is in the nature of an improvement on the invention disclosed in the co-pending application of W. I-I. Reichard, Ser. No. 70,259, led March 23, 1936.

Various other characteristic features, advantages and objects of the present invention will be evident from thek illustrated embodiments shown in the accompanying drawings in Which:-

Fig. 1 is a schematic circuit diagram shown for the purpose of illustrating the theory of operation of the present invention.

Fig. 2 is a diagrammatic representation o-f the parts and circuits constituting the vehicle equipment and trackway equipment, these parts and circuits being shown in a manner to facilitate in an understanding of the invention, rather than to show'the particular construction and arrangement of parts-preferably employed in practice. y

` General operation Referring to- Fig. l, alternating current energy generated 4aboard the Vehicle is transmitted through transformer TF3, through car carried inductors I1 and I2 in Aseries with condensers C3` and C4. ary windings of transformer TF3, inductors I1 and I2 and condensers Cfvan'd C4 is tuned to the source ofalternating current frequency, which coil disposed along the trackway.

` The trackway coils 111 and Il2 are likewise provided with U-shaped laminated cores, these coresl being made of large size'becausethey are intended in themselves, without their coils, to effect the train carried circuit as will be described.

When the train passes the wayside control point, train carried inductor I1 links magnetically with wayside inductor I11 and train carried inducto-r I2 links magnetically with wayside inductor 112.` The wayside coils are connected together in a series circuit including condensers C13 and C14, which circuit is tuned Vby these condensers and the inductors of the coils to the 360 cycle current. l

The two train carried inductors and the two wayside inductors are so Wound and connected in their` respective circuits that current from the generator flowing through coilsI1 and I2 in series induces current in coils 111 and 112, which are connected in series aiding relation with respect to-each other in the circuit including the two primary windings of transformer T5113 in series. The current which ows through this series circuit,` including the primary windings of transformer TF13 in series, induces an E. M. F. in the secondary winding of transformer TF'13 and this E. M. F. in the secondary winding transmits current through the frequency Vdetermining device (to be later described) to the primary (right hand) winding of transformer TF12.

The above discussion relates to the transmission of alternating current energy from the Vehicle tothe wayside apparatus over a series circuit including the car carried and trackway inductors. It Vwill be seen that the car carried apparatus, including transformer TF3 and inductors I1 .and I2 in series, operate as a trans- The series circuit including 'the second-Vv mitter and the wayside apparatus, including inductors 111 and 112, together with transformer TF13, act as a receiver.

The frequency determining device permits 360 cycle current, or double this frequency (720 cycles), to be applied to transformer TF12, the secondary winding of which. applies energy of this selected frequency to a parallel circuit including the primary windings of transformer TF13 and inductors I11 and 112. The wayside inductor coils are thus energized in a mutually parallel and opposite sense for retransmission back to the train. The return transmission circuit is therefore balanced and non-interfering with respect to the wayside receiving circuitpreviously traced and including inductors 111 and 112 in series.

position, the wayside inductors 111 and 11.2 may be regarded as excited by the return current inde-` pendently ofthe initial exciting current present in these coils.k The trackway inductors thus parallel reexcited, induce currents in the car carried inductors, with the E. M. F.s set up in these car carried inductors mutually opposing and. therefore neutralized with respect to the original current supply, in the series circuit above traced. These E. M. F.s are, however, mutually aiding with respectto and cause current to flow in a circuit superimposed upon and balanced with respect to the above mentioned series circuit.`

At a particular instant, the currentH flow'in this superimposed circuit extends from the upper terminal of coil I1 and the lower terminalof I2 over conductors l and 2 in parallel, through the secondary windings of transformer TF3, in opposing relation with respect to the primary winding of this transformer, conductor 3, condenser C1 andwinding of transformer TF1 or condenser C2 and winding of transformer TF2 (in accordance with the frequency of the current), conductor 4 and through condensers C3 and C4 to the opposite terminals of coils I1 and I2, which coils may be considered to be erimposed current. 4

From the above discussion of the retransmission back to the car, it will be observed that transformer TF13 is the receiver of the original energy transmitted from the car, transformer TF12 is the transmitter for retransmitting energy back to the car in accordance with signal indications, as determined by the frequency determining device and that'transformers TF1 and TF2 are theV receivers for receiving this retransmitted energy in accordance with the frequency of the current retransmitted.

An analysis of the fundamental circuit shown in Fig. l and considering the above general description of the operation of this circuit, it will be observed that the car carried inductors and the trackway inductors act assimultaneous transmitters and receivers. With such an arrangement, two distinct transmission paths are established, one for transmitting the exciting current to the trackway signal location and the other for selectively transmitting the signal indication currents back to the train. The train carried inductors operate in one series circuit to deliver power to the wayside inductors, the way-z side inductors receive the power and transmit it over a series circuit to the primary of transformer TF13. The return transmission path may be regarded as originating in the secondary of transformer TF13, the power in the secondary winding of this transformer being transferred to the source of supthe parallel circuit including the secondary (upper) winding of transformer TF12 and trackway inductors 111 and 112. Therefore, the wayside inductors operate in parallel for the return transmission to induce currents in the car carried inductors, which currents flow correspondingly in parallel along conductors l-2, through the windings of transformers 'IF3 (in non-energizing relation), conductor 3 and the receiver back to the car carried inductors.

Detailed operation The detailed circuit arrangement shown in Fig. 2 comprises the same arrangement of the circuits shown in Fig. las applied to the car carried and trackway apparatus for transmitting clear, caution and stop indications from a Way- By virtue of the well knownprincipie of super-v vjiside location to Control green. red and yellow lamps on Ythe vehicle, as an illustration of how the circuits of the present invention operate to transmit Asuch indications. v j Y The'car carried and trackway inductors, the transformers Vand the condensers have been given reference characters corresponding with similar devices illustratedi'n Fig. 1. In addition to these devices, the frequency determining device has been illustrated in detail in Fig. 2 and comprises a Yfrequency doubling rectifier unit FD and a frequency determining relayFDR, operating as will be later described in detail. Relays GR and YR are illustrated for the purpose of explaining how clear, caution and stop indications are transmitted Yfrom the trackway to the car. In-this connection it will be assumed that relays GR and YR are vin their deenergized conditions when a stop indication is to be transmitted, relay YR is in its energized condition when a caution indication is to be transmitted and relay GR is in its energized condition when a clear indication is to be transmitted.

The symbols (-i) and used in Fig. 2 designate the positive and negative terminals respectively of a source of direct current, such as a battery or the like, and the circuits to which these symbols are applied have current flowing in them in a single direction, that is, from to The source of alternating current energy is illustrated in Fig. 2 asbeing connected to diagonally opposite points 0f a Wheatstone bridge, made up of inductors L1, L2, L3 and L4 (the primary windingA of transformer TF3) in the four arms thereof. The secondary winding of transformer TF3 is connected to the car carried inductors and the primary windings of transformers TF1 and TF2. The secondary windings of transformers TF1 and TF2 are tuned to 360 cycles and '720 cycles respectively by condensers C5 and C5, with the secondary terminals of these two transformers being lconnected to the inputA circuits of vacuum tubes TY1 and TY2.

It is proposed to use vacuum tubes of the mercury Vapor or gas filled type (one type being commonly known as a thyratron tube). Such a tube is rendered conductive, or fired, by an increase in its grid potenial above a critical value, and also has the characteristic feature that once fired by such an increase in grid potential, it remains conductive even after the grid potential is restored to normal, until the tube is deionized and quenched by interrupting its plate circuit or the like. The grids of these two tubes are normally biased negative by battery CB and their plate circuits are energized by battery BB. Although no source of current is shown connected to the filaments of these tubes, it will be understood dication.

that their filaments may be energized in any desired manner, such as from a battery or a rectied source of alternating current, which arrangement may also be used for applying the plate and grid potentials to these tubes.

Full wave rectifier RC is connected to opposite diagonal points of the Wheatstone bridge and to the Winding of slow acting relay S, Which relay in combinationl with caution relay CA and clear l relay CL, control the circuitsfor the indication lamps as will be described in detail.

Normal condition-The circuits of Fig. 2 indicate thatthe last signal indication received was a clear indication, this eifecting thepicking up of relay CL. With front contact 2| of relay CA open and with back contact 22 of relayCL open, the plate circuit of tube TY2 is opened, which extinguishes this tube. A stick circuit is completed for relay CL which extends from front con-v tact 23 of relay CL, back contact 24 of relay CA, upper winding of relay CL and lfront contact 25 of relay S, to

' With relay CA down and with relay CL up, a circuit is closed for lighting the green lamp which extends from through the lamp, front contact 26 of relay CL and back contact 21 of relay CA, to 4 Y The bias of signalling'vused in the present discontinuous type of system is one wherein the wayside control elements are made to tend normally to give a stop indication and wherein the giving of a caution or a clear indication is accomplished by overcoming the stop indication condition and substituting for it a less restrictive in- Accordingly, there is provided in the present circuit organization a tripping means which gives a'stop indication unless positively displaced by a caution or clear indication. The trippingmeans comprises a normally unbalancedimpedance bridge (shown in the upper left hand portion ofFig. 2) which, under normal conditions, permits alternating current appliedto one pair of diagonal or conjugate points of the bridge to flow'in the opposite pairV of diagonal or conjugate points of the bridge and thence through rectier RC and the winding of relay S. This arrangement permits uni-directional current to flow through thewinding of relay S for maintaining it in its picked up condition, as illustrated, as long as the impedance bridge is unbal-A mitting a stop indication to the vehicle when the s.; car passes the wayside control point.

The impedance bridge shown in the upper left hand portion of Fig. 2 is normally unbalanced, as above mentioned, and the trackway inductor circuit is open because front contacts 3l, 32, 33 and 34 of relays GR and YR are all in their open positions. When the car carried inductors pass over the trackway inductors, with the trackway inductor series circuit open as illustrated, the inductive reactance of the circuit, 'includingl the car carried inductors and the two windings of transformer TF3 in series, is changed to a value which substantially balances Vthe impedance bridge.

There appears to be several electrical and magnetic phenomena involved in thus changingthe inductive reactance of the L1 arm of the bridge when the car carried inductors-come into juxtaposition with the wayside inductors. According to one `theory of operation, the series circuit including inductors 11-12 and condensers C3-C4 is substantially resonantunder normal conditions, when the car carried inductors are not over the Wayside inductors, but when the vcores ofthe car carried inductors are magnetically coupled to the cores of thevwayside inductors, the resonant con- 1 dition of the circuit is destroyed and losses are reflected into Vthis series circuit so that there is a substantial change in the impedance of arm L4. Among other things, the presence of the cores of inductors 111 and 112 may be considered as changing the magnetic reluctance of the cores of inductors I1 and I2 respectively. The inductive reactancerof thecoils of the car carried inductors are therefore changed, thereby disturbing the critical -resonant condition of the seriescircuit sor that thev impedance of arm I.;1 `of the bridge changed to bring' the bridge substantially into balance, thus resulting in no current flowing in the circuit including rectiiier RC and relay S. This drops relay S and closes an'obvious circuit at its back contact 35 for lighting the redlamp.

It is well known that an impedance bridge effectsa great gain in sensitivity by detecting the percentage change in impedance of one arm, over l that obtained by detecting the magnitude change.

circuits for relays CA and CL at front contact 25. g

Which causes both of these relays to drop, so that an additional circuit is closed for the red lamp, extending through back contacts 21 and 26 of relays CA and CL respectively.

` An examination of the circuits, particularly the schematic of Fig. 1, will indicate that, although alternating current is applied to inductors I1 and' I2 over the above described series circuit, no current from this source reaches transformers TF1 and 'I'F2 for Voperating the tubes, because these latter transformers are in a neutral or mutually inert circuit which is not affected by current 'owing in the series circuit. Likewise, transformer TF12 of the wayside apparatus is not affected because it isina neutral or mutually inert circuit with respect to the series circuit including inductors 111 and 112 and furthermore because this series circuit is open at the open front contacts of relays GR andYR.

Clear indication-It will now be assumed that relay GR is up and relay YR is down for transmitting a clear indication to the Vehicle. The above described series circuit including the two windings of transformer TF13 and inductors 111 and I12 is now completed through front contacts 3| and 34 of relay GR. This closed circuit condition of the trackway inductors maintains the inductance bridge unbalanced when the car carried `inductors `pass over the trackway inductors Y and the power transmission channel from the car to the trackway is terminated and turned back upon itself for-transmittingback to the car over the superimposed balanced parallel channel.

The current which is retransmitted back to th-e car and which flows over the parallel circuit including inductorsv Il-I2 and the two windings of transformer TF3, flows through these two windings in Vopposite directionsfrom that which flows through the windingslnthe series circuit, so that currentin this parallel circuit has no effect on the inductance in arm L4 Yof the bridge.

It will be understood that the unbalance of arm L4 permits current from the alternating current source to flow through rectifier RC and relay S for maintaining this relay` energized. It will further be understood that the discussion relating to the reection of losses fromthe trackway inductors to the car carried inductors is merely a theoretical explanation of the phenomena to which the results obtained inV practice are attributed. f

With the series circuit of the trackwayinductors complete when vthe car carried inductors pass over the trackway inductors, the disturbance of the resonant condition of the car carried inductor series circuit and the reflection of energy losses is small, so that there is onlya small change in the impedance reflected into the single Winding rof transformer TF3, which winding is arm L4 of the bridge. The operating y characteristics of relay S` and the constants of the different arms ofthe impedance bridge are so chosen, that this rela-y tively small change in impedance'in arm L4 is not suicient to bring the bridge into balance to the extent that relay S is released;

The non-effective condition of the cores of inductors I11 and 112, when the. closed series circuit exists, including the coilsr of these inductors, is attributed, among other things,l toythe choking efectof the circulating current in these coils, which tends to reduce the magnetic flux of the cores of the inductors, when these cores come into close relation with each other and thus reduce the influence on the car carried inductors, previously mentioned when th-e series circuit including the wayside inductors is open. There may be other contributing factors to be considered in a precise theoretical analysisof the operation above discussed, but the general effect is that closing the series circuit including inductors vI11-'-I12 renders these inductors ineffective to bring the impedance bridge into balance for dropping relay S. Y

Thus, in accordance with the above discussion, with relay GR picked up, current fromthe 360 cycle source on the car is transmitted from the series circuit, including inductors I1I2, Vtor the series circuit including inductors 111-112, the latter circuit including the two primary windings of transformer TF13 in series aiding relation. The secondary winding of transformer TF13 is connected through frequency doubler FD to the primary winding of transformer TF12. By means of this frequency doubler, each half-cycle ofthe alternating current flows through the primary (right hand) winding of transformer TF12 in the same direction. K

For example, when the upper terminal of the left hand winding of transformer TF13' is current flows from this terminal, rectifier 36, back contact 3l of relay FDR, downward through the right hand winding of transformer TF12, back contact 38 of relay FDR and rectifier 39 to the lower terminal of the left hand winding of transformer TF13. When the lower terminal of the secondary winding of transformer TF13 is (+4), then current yows from this terminal, through rectifier 28, back contact 31 of relay FDR, downward through the right hand winding of transformer TF12, back contact 3B of relay FDR and rectifier 29 to the upper terminal of the secondary winding of transformer TF13.

Current of a frequency double that of the 36,0 v

cycle current received from the vehicle (720 cycles) is thus induced into the left hand winding of transformer TF12 and applied to the above described parallel circuit, including the primary windings of transformer TF13 in opposing relation and the windings of inductances 111 and I111 in opposition.

Alternating current of '720 cycles is thus applied to the channel circuit leading back to the train, received by the parallel circuit including the car carried inductors and the windings of transformer TF3 in opposing relation and applied to the circuits leading to transformers TF1 and TF2. The primary and secondary windings of Vtransformer TF1 Vare tuned/to 360 cycles, therefore the 720 cycle current is not effective to fire tube TY1. The primary and secondary windings of transformer TF2 are tuned to 720 cycles, so that sufficient potential is impressed across the terminals of the right'hand winding of transformer TF2 and thus to the input circuit o tube TY2 for firing this tube.

Current vnow iiows in the plate or output circuit from the (-|)V terminal of battery BB, frontcontact v4I of relay'S, back contact 22 of relay CL, lower winding of relayY CL and plate-filament circuit of tubev TY2 to the terminal of'battery BB. Relay CL is picked up and the above "described stick circuit is completed for maintaining this relay picked up until dropped in response to a caution ora stop indication. 'I'he circuit is completed for lighting the green lamp, which circuit was previously traced. Tube TSE'2 isof course extinguished, as soon as relay CL picks up its contact finger 22.

Caution indication-With relay YRr picked up and relay GR down, thesarne operation is effected with respect to maintaining the impedance bridge unbalanced and with respect to the transmission from thevehicle to the trackway and the retransmission from the Vtrackway back to thervehcle. This is because frontA contacts 32 and 33 of relay YR complete the same circuits as contacts 3l and 34 of relay GR, previously described. However, with relay GR downand relay YR up, a circuit is closed for picking up relay FDR extending from v(-{-), back contact 42 of relay GR, front contact 43 of relay YR and winding of relay FDR to With relay FDR picked up and the consequent shifting of contacts 3l and 38, frequency doubler FD is removed from the circuit and the left hand winding of transformer TF13 is connected directly to the right hand winding of transformer TF12. This effects the retransmission of 360 cycle current back to the car, where it is received and caused to fire tube TY1 by means of the circuit tuned to the 360 cycle current, which circuit includes transformer TF1.

Current now ows from the (-4-) terminal of battery BB, front contact 4l of relay S, back contact 2| ofl relay CA (and in multiple therewith, front contact 22 of relay CL if relay CL is up), lower Winding of relay CA and plate filament of tube TY1 to the terminal of battery BB. Current in the plate circuit of tube TY1 picks up relayCA and drops relay CL (if up) by opening its stick circuit at back contact 24. 'Ihe dropping of relay CL closes a stick circuit for relay CA extending from back contact 23 of relay CL, upper winding of relay CA, front contact 43 of relay CA and front contact 25 of relay S, to

The opening of the plate circuit of tube TYl at back contact 2| of relay CA and at front contact 22 of relay CL extinguishes this tube.

Acircuit is now closed for lighting theyellow lamp which extends from (-1-), through the yellow lamp, back contact 44- of relay CL and front contact 21 of relay CA, to Relay CA, when picked up, remains stuck up until a stop or a clear indication isreceived on the train. The operation for dropping relay CA in response to a stop or a clear indication will be understood by analogy to the above descriptionV relating to the dropping of relay CL in response to a stop or caution indication.

.Relays CA and CL are interlocked so that each sticks through a back contact of the other. Furthermore, since the tubes are of the gas filled or arc discharge type, the picking up of relay CL andthe dropping of relay CA in response to a clear signal opens the plate circuit of the amplifier which effected this relay operation, in order to quench the tube. Likewise, the picking up of relay CA and the dropping of relay CL in response to a caution signal opens the plate circuit of the amplifier which effected this operation, in order ,to quench the tube.

From the above it will be seen that the unbalance of the impedance bridge is thrown in or out by relays YR and GR, with the` frequencies of the current retransmitted back to the train during this unbalance being controlled by these relays in accordance with clear and caution signals.

Although not shown or described, it will be understood that the relays on the car may be used for providing the desired safety of train operation, such asthe partial or full application of the brakes, providing an audible Warning signal or the like, all as shown in detail in the above mentioned prior application of W. H. Reichard, Ser. No. 70,259.

From the foregoing explanation of the opera-- tion of the circuit disclosed in the accompanying drawings, it will be seen that a simple and ecient system of intermittent inductive train control may be constructed in accordance with this invention, which satisfies lthe requirements and conditions for practical operation and which has the various important features and advantages pointed out. n l y Having now particularly described the invention and in what manner it is to be performed, what I claim isr- 1. In an automatic train control system, a car carrying an inductor, a trackway having an inductor, means including said inductors for transmitting current of a first frequency from said car to said trackway, and means including said inductors for'transmitting current of a second frequency derived from the current of said `first frequency from said trackway to said car.

2. In an automatic train control system, a car carrying an inductor, a trackway havingA an inductor, means including said` inductors for transmitting current of a first frequency from said car to said trackway, and means including said inductors for selectively transmitting current of said first frequency or of a second frequency derived fromthe current of said first frequency from said trackway to said car.

3. In an automatic train control system, a

car carrying an inductor, a trackway having anv inductor, means including said inductors forV transmitting current of a first frequency from saidcar Ato said trackway, and means including said inductors for selectivelytransmitting current of Asaid yfirst frequency or of a second frequency derived from the current of said rst frequency from said trackway to said car in accordance with trafc conditions along the trackway, y

4. In; an automatic train control system, a car carried inductor, a trackway inductor, means including said inductcrs for transmitting current of a first frequencyV from said car to vsaid trackway, means including said inductors for selectively transmitting current of said first frequency or of a second frequency derived from the current of said iirst frequency from said trackwayto said car in accordance with traffic conditions along the trackway, and car carried means selectively responsive to said frequency.

n5. In an automatic train control system, a car carried inductor, a trackway inductor, means including saidrinductors for transmitting current of a first frequency from said car to said trackway, means including said inductors for selectively transmitting current of said rst frequency or .of a second frequency derived from the current of said first frequency from said trackway to said car. in accordance withtraifc conditions along thetrackway, and car carried means-selectively responsive to said frequency and the absence of both said frequencies.

6. In an automatic train control system; the combination with car carried apparatus includimg a plurality of thermionic devices land a plurality of relays; a source of alternating current of a first frequency on said car; trackway controlled means for receiving said iirst frequency and translating it., into a second frequency; trackway controlledmeans for transmitting said first, said secondl or neither of saidfrequencies to saidl car; a Wheatstone bridge network on saidcar; means responsive to the reception of said first or saidsecond of said frequencies for causingsaid thermionic devices to selectively operate Va rst or a second of. said-relays respectively; and means `controlled by said Wheatstone bridge network for controlling the operation of a third one of said relays when neither of said frequencies is received.

'7.,In an'automatic train control system; a plurality of car carried electro-responsive devices; a pair of car carried inductors and a pair of wayside inductors; meansA for transmitting current `.impulses of a first frequency from the car to the wayside through the mediumy of said car carried and said wayside inductors; and means controlled by traflic conditions along the wayside for selectively transmitting current impulses of said first frequency or a second frequency derived from said car-carried current from the Wayside to the car through the mediumA of said car carried and said wayside inductors, whereby said Aelectro-responsive.devices are selectively controlled in accordance with traffic conditions along the wayside.

8. kIn an automatic train control system; a plurality of car carried electro-responsive devices;fa. pair of car carried inductors and a pair of wayside inductors; means for transmitting current impulses of a first frequency from the car to the Wayside through the medium of said car carried and said wayside inductors; and means controlled by traflic conditions along the Wayside for selectively transmitting current impulses of said first frequency or a second frequency, and each derivedfrom said rst frequency current from the car, from the wayside to the car through the medium of said car carried and said wayside inductors, whereby said electro-responsive devices are selectively controlled in accordance with` traiiic conditions along the wayside.

9. In an automatic train control system; a car carried source of alternating current; a car carried inductor unit including combined transmitting and receiving inductor coils, said coils being balanced with respect to the direct effect of said source; an inert wayside inductor unit Y including a pair of inductor coils for receiving energy from said source and for selectively en- Y ergizing the car'carried inductor coils with alternating current of the frequency of said source or a multiple thereof; and trailic controlled means for determining the'frequency with which said car carried inductor coils are'to be energized.

10. In an automatic train control system; a car carried source of alternating current; a car carried inductor unit including combined transmitting and receiving inductor coils, said coils being balanced with respect to the direct effect of said Source; aninert wayside inductor unit including a pair of inductor coils for receiving energy from said source and for selectively energizing the car carried inductor coils with alternating current of the frequency of said source or a multiple thereof; tramo controlled means for determining the frequency with which said car carried inductor coils are to be energized; and car carried indicators selectively controlled by said car carried inductor coils.

11. In an vautomatic train control system; a iirst car carried inductor winding and a rst coacting wayside inductor windingforming a first inductive coupling; a second carcarried inductor winding and a second coacting wayside inductor winding forming a second inductive coupling; a connecting circuit on the car which combines the inductive eiects of said rst and second car carried windings in a series circuit for the transmission of signalling current in one direction between the car and the wayside and in a parallel circuit for the transmission in the reverse direction; 'a Wheatstone bridge circuit on the car; an electroresponsive device connected to said Wheatstone bridge circuit; means controlled by traiiic conditions along the wayside and rendered effective during the coactive coupling between said car carried and said wayside windings for applyingk or not applying signalling current to said series circuit; and means controlled by the presence or absence of 'signalling current in said series circuit during said coactive coupling for unbalancing or balancing said Wheatstone bridge circuit, whereby said electro-responsive device is selectively controlled. 3

12. In an automatic train control system; a car carried source of alternating current connected to a pair of conjugate points of Va Wheatstone bridge circuit; a first electro-responsive device connected to an opposite pair of conjugate points of said Wheatstone bridge circuit; a pair of carl carried inductor coils supplied with alternating current from said source; a car carried signal receiving circuit utilizing said pairl of car carried inductor coils; a second electro-responsive device connected to said receiving circuit; circuit means connecting said'receiving circuit and said source to said pair of car carried inductor coils so as to prevent alternating current from said source from directly affecting Said receiving circuit; a pair of wayside inductor coils; circuit means connecting said wayside inductor coils in parallel for the transmission of signalling current in one direction from or to the train and in series for the transmission of signalling current in the reverse direction; means controlled by the presence -or absence of signalling current in the series connection of said wayside inductor coils for balancing or lunbalancing said Wheatstone bridge circuit, whereby saidrst electro-responsive device is selectively controlled; and means controlled by signalling current in the parallel connection Aof said wayside inductor coils for affecting said receiving circuit, whereby said second electro-responsive device is selectively controlled.

13. In an automatic train control system; a car carried source of alternating current connected to a pair of conjugate points of a Wheatstone bridge circuit; a'rs't.electro-responsive device connected to an opposite pair of conjugate points of said Wheatstone bridge circuit; a pair of car carried inductor coils supplied with alternating current from said source; acar carried signal receiving circuit utilizing said pair of car carried inductor coils; a second and a third electro-responsive device connected to said receiving circuit; circuit means connecting said receiving circuit and said source to said pair of car carried inductor coils so as to prevent alternating current from said Source from directly affecting said receiving circuit; a pair of wayside inductor coils; circuit means connecting said wayside inductor coilsin parallel for the transmission of signalling current in one direction from or to the train and in series for the transmission of signalling current in the reverse direction; means controlled vby the presence or absence of signalling current in the series connection of said wayside inductor coils for balancing or'unbalancing `said kWheatstone bridge circuit, whereby said rst electro-responsive device is selectively controlled.; and means controlledV by signalling current'in the parallel connectionof said wayside inductor coils for aifecting said receiving circuit, 'whereby said second and said third electro-responsive devices are setwo wire circuit; means associated with said secondary winding for receiving current impulses of one frequency from said two wire circuit, translating them into current impulses of another frequency and transmitting current impulses of said another frequency oversaid superimposed circuit; and train carried, means controlledby said another frequency in saidsuperimposed cirfv the two wire circuit in parallel; a transformer along the wayside including primary and secondary windings, said mid-point connection at one end being connected to the mid-point of sai-d primary winding and said primary winding being included in said two wire circuit; means associated with said secondary winding for receiving current impulses of one frequency from said two wire circuit, translating them into current impulses of another frequency and transmitting current impulses of said one and said another frequency over said superimposed circuit; and train carried means controlled by said one and said another frequency in said superimposed circuit.

16. In an automatic train control system; a car carried source of alternating current; car carried inductor coils connected in series in 'a series circuit energized from said source; a normally deenergized clear indication receiving circuit on the car connected to said coils in a parallel circuit; said parallel and said series circuits being mutually inert; and trackway means receiving energy from said coils when said series circuit is energized for applying current of a frequency diierent from said source to said coils and said parallel circuit only if a clear condition is to be transmitted from the trackway to the car, whereby said clear indication receiving circuit is energized.

17. In a system of intermittent inductive train control; in combination with a car carried receiving circuit into which a momentary voltage of a first or a second frequency is induced from the trackway; a first and a second electronic tube coupled to said circuit, said first tube rendered conductive by a Voltage of said first frequency in said circuit and said second tube rendered conductive by a voltage of said second frequency in said circuit and each tube remaining conductive until quenched by means independent of said circuit; indication means controlled by said tubes; and means controlled in response to each tube becoming conductive for quenching this tube and conditioning the other tube for response to the voltage in said circuit which renders the other tube conductive.

18. In an automatic train control system; a pair of wayside inductors; a train carrying -a periodically varying current transmitter; a first means connecting the two wayside inductors in 'series with each other in a first circuit for'receiving inductively current from said transmitter; a second means including said first meansv connecting said two inductors in parallel with each other in a second circuit for inductively retransmitting current from said inductors to said train; means controlled by traic conditions in advance for deriving current from said first circuit, determining its frequency and retransmitting it over the second circuit to said train; and receiving means on said train responsive to current in said secon-d circuit and the frequency of said current and non-responsive to the current in said first circuit.

WINFRED T. POWELL.

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