US2859335A - Track circuit responsive to varying train shunt - Google Patents

Description

Nov. 4, 1958 IM. A. scHr-:G

y TRACK CIRCUIT RESPONSIVE TO VARYING TRAIN SHUNT Filed Sep't.. 9, 1954 3 Sheets-Sheet 1 TRP SWITCH AND/0R SIGNAL CONTROL INVENTOR.

M. A. SCHEG mmw HIS ATTORNEY NOV. 4, 1958 M A, SCHEG 2,859,335

TRACK CIRCUIT RESPONSIVE TO VARYING TRAIN SHUNT Filed Sept. 9, 1954 FIG. 2.

l s sheets-sheet 2 3 TRACK RA|LS\ [2 /3 I/CABLE FROM TRACK RAILS TO RELAY LOCATION I Y 28 SWITCH ANO/OR SIGNAL CONTROL IN VEN TOR.

Y MA. SCHEG 7AM/fsw HIS ATTORNEY M. A. SCHEG Nov. 4, 1958 TRACK CIRCUIT RESPONSIVE To vARYING TRAIN sRuNT l Filed sept. 9, 1954 3 Sheets-Sheet 3 om o om om o. o. n n m I I I l I IIN IIIII Ill l I I l I I I| log. ioo 5mm T m A l I l l Ill |l|| IiIIl llllllliloo. 5-5i d n m U l I l l I I I I I I I I l I I I I l l I I l I I. I 'All l. OO C @z xo; 1mmw| M L E R HIS ATTORNEY nited States Patent O TRACK CIRCUIT RESPONSIVE T VARYING TRAIN SHUNT Marcian A. Scheg, Rochester, N. Y., assgnor to General Railway Signal Company, Rochester, N. Y.

Application September 9, 1954, Serial No. 454,956

13 Claims. (Cl. 246-41) This invention relates to direct current track circuits for railroads, and more particularly pertains to means for increasing the sensitivity of such track circuits in response to train shunts.

The direct current track circuit is in general use on rail- :roads and comprises a source of direct current connected in series with a limiting resistor across the rails at one end fOf the track section with a track relay connected across the rails at thel other end of the track section. The limiting resistor is so adjusted as to provide that the ktrack relay is normally energized; but when a train enters the track section and shunts the track rails, the interrail potential is reduced to a value below the drop away valuey tof the track relay.

It has been found that there `are various conditions under which the usual track circuit organization above described fails to be suiiiciently sensitive to train presence .as to maintain a track relay dropped away under all conditions of train presence. This is because the running :surface of the track rails may sometimes be rusty, or may be covered with a thin layer of sand or other foreign material. It has been found in testing such track circuits where foreign material is located on the rails, that a vmoving train or car will provide a variable resistance .shunt even though it may not at all times provide a shunt :sufficiently low in resistance as to cause the release of the Iconventional track relay.

Thus, the present invention proposes to provide a track 'circuit organization which is responsive both to regular :resistance train shunts and to variable resistance train shunts. It is proposed to organize the track circuit so :that it will be selective between the changes in shunting :current with respect to the entrance and exit of a train 4to permit the rapid establishment of normal conditions `upon the leaving of the train. This is particularly de- :sirable in connection With automatic switching in classilication yards where the headway between successive cars :is dependent upon the restoring time following the exit tof a car.

Other objects, purposes, and characteristic features of the present invention will in part be obvious from the accompanying drawings, and will in part be more speciiically described hereinafter.

In describing the invention in detail, reference will be made to the accompanying drawings in which:

Fig. l is a diagrammatic view of a track section and associated apparatus to constitute one form of track circuit embodying the present invention to make the track circuit sensitive to varying train shunts, which form is more particularly applicable to short track circuits;

Fig. 2 is a modification of Fig. l to make the track relay sensitive to variations in train shunt but omitting .the separate impulse responsive relay shown in Fig. l by `applying impulse energy directly to the track relay; and

Figure 3 is a graph of the relay current and ballast resistance characteristics with a track circuit organized as illustrated in Figs. l and 2.

For the purpose of simplifying the illustration and fa- 2,859,335 Patented Nov. 4, 1953 ice shown diagrammatically and certain conventional illustra-y tions have been employed, the drawings having been made more with the purpose `of making it easy to understand the principles and mode of operation, than with the idea of illustrating the specific construction and arrangement of parts that would be employed in practice. Thus, the various relays and their contacts and associated devices are illustrated in a conventional manner.

With reference to Fig. l, a typical track section having rails 2 is shown as being separated from adjoining track sections by suitable insulated joints 3. It is assumed that this typical truck section is a detector track section that may include a track switch (not shown). The length of such a detector track section may vary within reasonable limits, and it is assumed that all of the apparatus including the track relay and the track battery may be located in a single wayside case and have the opposite ends of the track section connectedto it through suitable cable leads which have been illustrated in the drawing by a suitable legend.

A transformer TF has a primary Winding 5 which is connected to the track section at the right-hand end through cable leads 6 and 7 in series with a limiting resistor R1 and a source of direct current shown as a battery B.y Another primary winding 10 on the transformer TF is connected through cable leads 11 and 12 directly across the track rails at the'entering or relay end of the track section, but with a polarity such that the flux produced by it is in opposition to the flux produced by winding 5. The transformer TF is so constructed that it has a center tapped secondary winding across which is connected a capacitor C3. Associated with the outer connections to this secondary winding 15 are two rectifier or asymmetrical units 16 and 17. The connections to the secondary winding 15 through these rectifier units will be described later.

A track relay TR is connected through its own front contact 18 and resistor R2 across the terminals of the track cable leads 11 and 12. It should, of course, be understood that the resistance values of the track relay TR, limiting resistor R1 and adjustable resistor R2 may be suitably selected in accordance with the conditions of practice. But it has been found in various Cases that short detector track sections may have ballast characteristics which will vary from ten ohms to substantially infinity; and with such a track section, it is convenient to employ a track relay having approximately four ohms in its windings in combination with an adjustable resistor R2 having approximately one hundred ohms. The resistor R1 may be in the order of two or three ohms. With this organization, the battery B may have in the order of nine to ten volts which will provide sufficient interrail potential to in most cases breakdown the usual film or glaze found on the running surface of the rails.

The particular points of adjustment for the resistors R1 and R2 will, of course, depend upon the actual conditions encountered in a particular section of track. The track relay TR is normally energized through its stick contact 18, but once a train enters the track section and effectively shunts such atrack relay, it releases and cannot again pick up until the impulse relay IR is picked up closing front contact 19. Thus, there is a check on the response of the impulse relay IR in that it must be operated everytime the track relay TR is picked up. The track relay TR has a front contact 20 which applies energy (-1-) to a back point of contact 21 on the impulse relay IR so that energy is fed to a track repeating relay TRP while both of these contacts 20 and 21 are closed. This track repeating relay TRP is normally energized and has contacts and a relatively lou/number of turns,: whilethetotherrv winding H has a relatively high resistance-anda relatively..` 1 high number of turns.l Forexample, onesuitable-.frelayi has a winding L with/six hundredy turns of-wire having-A a total resistance of 1.25 ohms,V and'2 the other1winding H has nine thousand Aturns and a resistanceof*.260rbhmsa4 `is so as Ato make the control'ofrtherelay quickeruporr release when the-windingvLV is the onebeing--deenergized-l This upper.windingvL-isshunted by a.v capacitor Cito l reduce the arcingat the contactsrcontrolling it and to give certain timing characteristics-laterrdescribecL For similar reasons,:a capacitor C2 andY associated resistorRS- are associated with the-lower windingH.A This capacitor CZand resistorV R3 are connected across the lower winding H through' front contact 220i relay TR; and when they trackgrelay TR is released back-contact '22 is closed so as to dissipate any energy stored in the capacitor C2.

When` the-track relay TR-is picked up closing front contact 25j a circuit is completed for the energization of the impulse relay IR through both of its vwindings in series .with the-rectier or asymmetrical unit 17. This circuit with-thelrectier 17 thuspoledl permits impulses to passth'rough the vimpulse relay IRfupon azdecreasein. the current. in primary winding I() and an. increaseV in current inthe primary windingS as would occur upon the entrance of `a, train.

When the-trackrelayTR is released, back contact 26 of track relay TR'is closed to complete -a circuit throughv rectifier liand relay IR to permit impulses to flow upon rises in currentin the primary winding It) and decreases in the current inprimary winding 5 as would occur upon the exit of a train.

It is `believed Vthat further description of the track circuit organization willl best be. understood by considering the condition of operations.

Operation Under normal, conditions, the limitingresistors R1 and; R2 are adjusted so that the operating current of the track relay TR is slightly aboveitsj pick up value. This may be in the orderof .100 amperes assuming that a particulartrack relay is used.

With referenceto Fig.: 3, it will be seen that the increase in the ballast resistance will cause less leakage between the track rails so that -thelpotential drop in the limiting resistor R1 will decrease. This in'turn causes a slightly rising interrail potential VsoV that the relay current will increase; butsince thel resistor R2 Vis suitably adjusted to includea substantialramount of resistance such as in the orderrof'iiftyk ohms, the relay current does not become excessive as indicated by the curve. otFig. 3. This means that regardless ofgwhether theballast vresistance is relatively low or high thetrackrelay TR is not materially over energized and is sensitiveto the en trance of a train into the section to reduceit's current below the drop away value and thus register the trainpresence. In this connection, it should also be noted` that the provision of a relatively high-:resistance-in resistor R2 permits the interrail potential to be maintained under normal condition at a substantial value above what is known as the normal ionization or breakdownV voltage of the lm that may form on the track rails.

The normal interrail potential also causes a normall current to ow through the primary winding 10 ofthe transformer TF with the complete current supplied to the track circuit ilowing through the transformer winding 5. The number of turns on each of these windings issoselected that the magnetic lux produced by the nor mal current in the winding 10 is in predominance7 because these two windings 5 and 10 are connected to their output. terminals in such a way that the flux produced by these two windings are in opposition. Thus, the net uX is of a particular value depending upon the relative currents passing through these two windings.

When a train enters the track section and shunts the track rails with a usual or regular shunting effect, the current is substantially reduced in the winding 10 and the current/is substantially increased in the winding 5. This means that the flux produced by the winding 5 predominates .over that produced by the winding I0. In this way, the fluxis caused to reverse its direction in the transformer TF and in Vthus passing through a zero value produces-.the greatestvinductiveV effect on the secondary winding 15. This inductive pulse will ow from the righthand terminal of secondary winding I5 through the rectier unit 17, front contact 25 of track relay TR, lower winding of relay IR, upper winding of relay IR back to the center tap of winding 15 oftranstormer TF.

Such an inductive pulse causes the impulse relay IR to pick up opening back contact to release the track repeater relay TRP even more quickly than the track relay TR can release and open. front contact 2i). In any event, the presence of such a train causes the track relay TR to release opening front contact 2h so that the track.

relay TRP remains released opening its .contact 27 regardless of whether or not the impulse relay IR remains picked up due to varyingshunt values as the train Vprogresses through. the section. It will be understood that with .the two windings of the impulse relay IR in series the; relay is verysensitive andalso highly inductive.

In addition, the capacitor CI is connected across the winding L of the impulse relayIR; and also the capacitor C2 withits resistor R3 is connected across the lower winding H of relay IR through front contact 22 of track relay TR. These capacitors across the two windings of the relayV IR cause it to have slow releasing characteristics.

But if the train-has a regular shunting eect and causes the release of the track relay TR in the usual way, then the front contact 2t) holds open the energizing circuit for the repeater relay TRP, so that it is kunnecessary for the releasing characteristics of the relay IR to be slow under such circumstances. Forvthis reason the control of the windings of relay IR may be shifted .by the track relay -in preparation for the subsequent leaving of the train from the track section. This is effected by contacts 22, 25 and 26. More specifically, the opening of front contact 22 disconnects the capacitor C2 from across the lower winding H of impulse relay IR and completes a circuit through back contact 22 so that this capacitor C2 may complete its discharge through the resistor R3. The opening of front contact 25 disconnects the lower winding H of relay IR from the transformer; while the closure of back contact 26 connects the upper winding L to the transformer through the rectier unit 16.

If the train provides the regular shunting effect, then there are ysubstantially no variations in thetrack current and thus there is substantially no impulseenergy supplied to the relay IR which condition is proper since the track relay TR is steadily dropped away.

However, sho-uld the train shunt be of an insuicient value to release the'track relay, it has been found Vthat it will be of a more or less continuously varying value. In other words, the current supplied from the track battery will be continuously varying and the interrail potential at the relay end of the track section will lbe continuously varied. During this continuously varying condition of the trainshunt value, the changing current through the primary windings 5 and'lt) provides a continuously Varying output through the rectifier unit I7 to the two windings of the relay IR connected in series. This is because under these conditions it is Aassumed that the track relay' TR remains picked up. With this inductive output supplied to the relay yIR.through its high- IR is relatively slow to release and particularly so with the capacitor C2V and resistor R3 connected across its lower winding. This means that a more or less continuously varying train-shunt will cause the impulse relay IR to remain steadily picked up and thus maintain the track repeating relay TRP steadily released in spite of the fact that the track relay TR may not have been released upon the entrance of a train.

However, returning to the more usual condition where the track relay TR is released by the regular trainshunt, the variations in the train-shunt under such conditions while the train is passing through the section has less significance but they may be fed through the rectier unit 16 and back contact 26 of track relay TR, upper winding of relay IR to the mid tap of the secondary winding 15. Whether or not the impulse relay IR picks up under such conditionsl is relatively unimportant sincel the track relay TR is maintained steadily released.

But when the train leaves the track section, the current in the winding 5 is substantially reduced and in the winding is substantially increased. This again causes a complete reversal of the net flux in the transformer TF so that a substantial impulse is supplied to the upper winding of the impulse relay IR through back contact 26 o-f track relay TR. This picks up the contacts of relay IR closing front contact 19 to shunt the stick contact 18 in the track relay circuit and allow the track relay TR to be picked up to a normal position. This opens the back contact 26 to disconnect the upper winding of the impulse relay IR from the transformer. This tends to quickly release relay IR since the capacitor C1 is relatively small because the resistance of the upper winding is relatively low. Also, since this upper winding has a relatively small number of turns, its inductive characteristics are considerably less than that of both windings in series, so that any impulse supplied to this upper winding tends to be dissipated relatively quickly. Thus, the impulse relay IR will release relatively quickly under these circumstances and close its back contact 21.

By way of comparison, impulse energy supplied to both windings in series may cause the impulse relay IR to remain picked up for a period in the order of .6 of a second, whereas an impulse of the same magnitude applied to only the upper winding may cause the impulse relay IR to remain picked up for only .2 of a second. This difference in time is of particular importance for detecto-r track sections used in automatic switching where .4 of a'second additional time may require an additional eight feet headway between successive cars being classied.

In this way, the exit effect of a car from the track section, provided that the impulse relay IR is actuated for a suicient time to pick up the track relay TR, but it does not remain picked up for an undue length of time delaying the desired switching operation.

Let us assume that the passage of a train through the track section takes place with the shunt resistance value failing to be suiiciently low to cause the release of the track relay TR. As above stated, such a rolling shunt even though on sand or other foreign material has a tendency to create a continuously varying resistance shunt so that the current values in the windings 5 and 10 will be continuously changing. For this reason, the out put of the winding is supplied through rectifier 17 to the two windings of the impulse relay IR in series (its most sensitive connection) and will thus cause the impulse relay IR to remain continuously picked up so long as the shunt is continuously changing. However, in this instance, the leaving of the train causes an increase in the flux produced by winding 10 and a ydecrease in the flux produced by winding 5. This change in net flux has an effect on the winding 15 to produce an impulse for flowing through the rectifier unit 16, but under the circumstances assumed back contact 26 is opened. The polarity connection of the rectifier unit 17 prevents the flow of such impulse through the windings of relay IR. Thus, the release of the relay IR under these assumed conditions is not materially atTected by the abrupt exit impulse, but has a release time dependent upon the magnitude of the preceding shunt resistance variations.

In this way, the track circuit organization responds both to regular train-shunts and to continuously varying train-shunts to provide a reliable detecting action; and such organization makes the response very sensitive and yet at the same time controls the response so that it does not unduly delay the desired operation upon the leaving of a train.

Figure 2 With reference to Fig. 2, the track section has the same connections to it from a battery B, transformer TF and track relay TR2. The relative adjustment of the batteries and resistor values for the track circuit are assumed to be exactly the same as discussed in connection with Fig. l.

The difference between Fig. 2 and Fig. l is that the impulse relay IR is eliminated and the impulse output of the secondary winding 15 of transformer TF is supplied directly to the lower winding of the track relay TR2. This track relay TR2 is of the polar biased type which responds to actuate its contacts only in response to current of a particular polarity. One such suitable relay is disclosed in the prior patent of Willing et al., No. 2,502,811 granted April 5, 1950. The upper winding of the relay TR2 is included in the connection directly across the track rails, so that this relay will respond to the usual train-shunt in the usual way.

However, the entrance of a train into the track section and its exit from such section will cause substantial impulses to be supplied to the lower winding of relay TR2 in a direction due to the connections of the rectiers 16 and 17 so asto produce a flux in the lower winding of the track relay TR2 in a direction to cause its contact 28 to release. It is noted that this impulse energy may be of any particular value to cause the release of the track relay TR2 since any over energization of this lower winding cannot possibly cause the picking up of the contacts because of the characteristics of such a polar biased relay. The capacitor C4 is connected in multiple across the relay TR2 to make the impulses more steadily effective.

It will be noted that a resistor R4 has been included in series with the rectiier unit 16 so that the impulse energy produced by the winding 15 upon lthe leaving of a train may be regulated to a value which does not cause a substantial over energization of the relay in the reverse direction. In this way, the track relay TR2 may respond to lits normal energization within a relatively short time after the exit of the train and pick up to its normal position.

Having described two forms of track circuit which are responsive to variations of train-shunt values as specific embodiments of the present invention, it is desired to be understood that these forms are selected to facilitate in the disclosure of the invention rather than to limit the number of forms which it may assume; and it is to be further understood that various modifications, adaptations and alterations may be applied to the specific form shown to meet the requirements of practice, without in any manner departing from the spirit or scope of the present invention.

What I claim is:

l. In combination with a section of railroad track insulated from adjoining sections, an impulse transformer having two primary windings and a secondary winding, circuit means including a source of direct current and a limiting resistor connected in series with one of said primary windings across the track rails at one end of said track section, a track relay having a frontxcontact,

circuit means for connecting said track relay across the rails at the vother end of said section through its own front contact, circuit meansfconnecting the other of said primary ,windings in opposition to said irst primary winding and acrossvthetrack rails at Ythe same end as said track relay connection, an impulse V,responsive relay, circuit means connecting `said impulse responsive relay to the secondary winding of said impulse transformer, circuit means includingafront contact of saidlimpulse responsive relay for at times shunting said front contact of said track relay, and signaling means controlled through a front contact of said trackirelay in Yseries with a back contact of said impulse responsive relay, whereby impulses of substantial magnitude are produced in said secondary winding upon abrupt changes in the interrail potential as causedby a train-shunt.

2. In'combinationfiwith a'section .of railroad track insulated. from adjoining sections, an; impulse transformer havingtwo primary windings. anda secondary winding, circuit/means including a battery and alimiting resistor connected in series with one of saidnprimary windings across the .track rails at one end of said section, a track .relay having a front contact and a limiting resistor, circuit means .for connecting said track relay and said limiting resistor through said front contact in multiple with the other primary winding and across the track rails at the -other end of said section, said other primary windingbeing connectedin opposition to said one primary-winding, an impulse responsive relay, circuit means including Vrectifier means for connecting said irnpulse responsive relay to said secondary winding for supplying impulses from said `secondary winding to said impulse responsive relay, alfrontcontact onssaid impulse responsive relay connected in multiple with the front contact of said track relay, aqrepeating relay connected through Va front contact of said track relay and `a back contact of said impulse responsive lrelay in series, whereby said track relay when -once released requires the response of said impulse responsive relay in order to be again picked up, and whereby the effective shunting of said track rails causes the reversal of the net magnetic iiux in said impulse transformer to cause a maximum impulse to be produced by the secondary of said transformer 4for actuation of said impulse responsive relay.

3. In combination with a section of railroad track insulated from adjoining sections, a track battery and a limiting resistor connected across the rails :atone end of said section, a track relay having a front contact and a limiting resistor, circuit means for connecting said track relay and said limiting resistor through said front contact across the rails at the other end of said section, an impulse transformer having two primary windings and a secondary winding, circuit means connecting one of said primary windings of said impulse transformer to the track rails adjacent the connection of said track relay so as to cause said impulse transformer to be responsive to changes in the interrail potential, circuit means connecting the other of said primary windings in series with the connection of said track battery to the rails of said track section, an impulse relay having two windings one highly inductive and one of relatively low inductance characteristics, circuit means including rectifier means and contacts of said track relay for connecting both of said windings of said impulse relay in series to the secondary winding of said transformer while said track relayis picked up but connecting only the winding of said impulse relay of relatively low inductance to the secondary winding of said transformer when said track relay is released, a front contact on said impulse relay connected in multiple with the front contact of said track relay, and signaling means controlled through a front contact of said track relay and a back contact of said impulse rel-ay in series.

4. In combination with a section of railroad track insulatedfrom adjoining sections, an impulse transformer having two oppositely connected primary windings and a s secondary winding, circuit means including a source of direct, current and a Vlimiting resistor connected in series with one yof said primary windings across the trackrails atone end of said.'section, a trackrelay having afront contact, a current,limitingrresistor,V circuit means forl connecting-saidtrack relay in series with said current limiting resistor through its ownjfront contact and across the rails at the other end of said section, circuit means connecting the other of said Vprimarywindings across the track rails at the same end of the track section as the connection of said track relay, said other primary winding producing,

pulse responsive relayitoy the secondary-winding of said impulse transformer, whereby the variations in the interrail potential caused by the passage of `a car provides said impulse responsive relay with pulses of direct current and whereby saidimpulse responsive relay receives a current pulse of substantial magnitude whenV the magnetic flux produced rby said one primary winding increases to predominate over'the magnetic flux produced by said other primary windingl causing the magnetization of the transformer to pass throughfa Zero'uX condition, and circuit means including atrackv repeating relay connected in series with'a'backcontact of saidimpulse responsive relay and a front conta-ct of said track relay.

5.' In combination with a section of railroad track insulated from adjoining sections,'an impulse transformer having two primarywindings and a secondary winding, circuit means kincludinga source of direct current and a limiting resistor connected in series Vwith one of said primarywindings across the trackrails at one end of said track section,` circuit means connecting the other of said prirnary-windings across the track rails at the other end of. said track section, a track relay having two windings, circuit means for connecting one of said two windings across the trackrailslat said other end of said section, and other -circuit means for connecting the other of said two windings tov said secondaryiwinding of said transformer in opposition to the connection of said one winding, and signaling means controlled through .la front contact of said track relay.

6. In combination with a section of railroad track insulated from adjoining sections, an impulse transformer having two primary windings and a secondary winding, a source of direct current and a limiting resistor, circuit means connecting said source of direct current and said limiting resistor in series with one of said primary windings across they track rails at one end of said section, circuit means connecting the other of said primary windings -across the track rails at the other end of said section, said connections of the primary windings to the track rails being with the opposite polarity so that the magnetic flux produced by the two primary windings are in opposition, a track relay of the polarized type having two windings and having contacts biased to one position, a current limiting resistor, circuit means connecting one of said windings of said track rel-ay through said current limiting resistor to said other end of said section with the proper polarity to normally cause the response of its contacts,

circuit means including rectifier units for connecting the` secondary winding of said transformer to the other winding of said trackrelay with a polarity in opposition to said one winding of said track relay, and signaling means controlled by contacts of said track relay, whereby said track relay normally has its contacts in actuated positions but its contacts are restored `to their biased positions upon the entrance of a train into said track section due to the shunting effect of the train across the track rails and the reduction of said energy in said one winding of said track relay and also due to variations in shunting effect causing pulses of energy to energize said other winding of said track relay.

7. A track circuit organization according to the preceding claim 6 in which a capacitor is connected across said other winding of the track relay whereby the impulses from the secondary winding of the transformer are rendered more continuously effective to cause the contacts of the track relay to be operated to their biased positions.

8. In combination with a section of railroad track having insulated joints separating it from adjoining sections, a source of direct current, a limiting resistor, an impulse transformer having a primary winding and a secondary winding, a circuit means connecting said primary winding and said source of direct current and said limiting resistor in series across the track rails of said section at one end, a track relay of the polar biased type having two windings, circuit means connecting one of the windings of said track relay across the track rails at the other end of said track section to thereby normally energize said track relay to actuate its contacts away from their biased positions, other circuit means including a rectifier unit from connecting the secondary windings of said transformer to the other winding of said track relay with such a polar connection as to tend to cause the contacts of said track relay to be restored to their normal biased positions when impulses are supplied to such other winding of said track relay when variations occur in the interrail potential, whereby variations in a train shunt are effective to cause the release of said track relay contacts to their normal biased positions, and signaling means controlled by said track relay.

9. In combination with a section of railroad track insulated from adjoining sections, an impulse transformer having two primary windings and a secondary winding, a source of direct current, a limiting resistor, circuit means connecting said source of direct current and said limiting resistor in series with one of said primary windings across the track rails at one end of the section,

whereby said impulse responsive means is rendered highly effective because of a reversal of ux in the transformer upon the entrance of a train.

10. In combination with a track section, a source of direct current connected across the track rails at one end of the track section, a track relay connected across the tra-ck rails at the other end of the track section, an impulse transformer having two primary windings and a secondary winding, an impulse relay, circuit means for connecting said primary windings in opposition, one of said primary windings being connected to the track rails at said one end of the track section in series with said source of direct current and the other of said primary windings being connected to the track rails at said other end of the track section, circuit means for connecting said impulse relay to the secondary winding of said impulse transformer, and electroresponsive means actuated in response to the actuation of either said track relay or said impulse relayA 11. In combination with a track section, a source of direct current, an impulse transformer having two primary windings and a secondary winding, circuit means for connecting said source of direct current to the track rails at one end of the track section through one of said primary windings, circuit means for connecting the other of said primary windings across the track rails at the other end of the track section, said other of said primary windings having a suicient number of turns to normally produce a predominant magnetic flux in said impulse transformer in opposition to the magnetic iiuX produced by said one primary winding, rectifier units, and electroresponsive means connected through said rectifier units to said secondary winding to be responsive to impulses induced in said secondary winding.

12. In combination with a track section, a source of direct current, an impulse transformer having two primary windings and a secondary winding, circuit means for connecting said source of direct current to the track rails at one end of the track section through one of said primary windings, circuit means for connecting the other of said primary windings across the track rails at the other end of the track section, the two primary windings of the impulse transformer being connected to produce magnetic tiuxes in opposition to each other, and electroresponsive means jointly controlled by the interrail potential at said other end of the track section and -by impulses produced in said secondary winding by the passage of a train shunt through the section for governing railway apparatus.

13. In combination with a track section, a source of direct current connected across the track rails at one end of the track section, a track relay connected across the track rails at the other end of the track section and normally energized by the interrail potential, impulse responsive means including an electromagnetic relay and an impulse transformer having two primary windings and a secondary winding, circuit means for connecting said primary windings in opposition with respect to each other and for connecting one of them to be energized dependent upon the interrail potential at the track relay end of the track section and for connecting the other of them to be energized in series with current from said source at the source end of said track section, other circuit means for operatively connecting said electromagnetic relay to said secondary winding, and electroresponsive means jointly controlled in response to the actuation of said track relay and in response to the actuation of said impulse responsive means.

References Cited in the file of this patent UNITED STATES PATENTS 1,630,727 Woodbridge May 3l, 1927 1,777,651 Peter Oct. 7, 1930 1,812,208 Lazich lune 30, 1931 2,133,177 Pflasterer Oct. 11, 1938 2,135,527 Powell Nov. 8, 1938

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