US2817009A - Track circuit for railroad - Google Patents

Description

Dec. 17, 1957 o. s. FIELD 2,317,009

TRACK CIRCUIT FOR RAILROAD Filed Nov. 9, 1953 3 Sheets-Sheet 1 mmmmrgflmmm/ fiy aouc OF CoNDu ING R assa BENEATH E. Qmm. R

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IN V EN TOR.

V 10 \J 3 M. l 7 ms ATTORNEY Dec. 17, 1957 o. s. FIELD 2,817,009

TRACK CIRCUIT FOR RA ILROAD Filed Nov. 9, 1953 5 Sheets-Sheet 2 [-INSULATION O N I l FIG. 6.

NSULATION Pie-.7.

INVENTOR.

HIS ATTORNEY Dec. 17, 1957 o. s'. FIELD 2,817,

TRACK CIRCUIT FOR RAILROAD Filed Nov. 9, 1953 5 Sheets-Sheet 3 l I I 57 I SIGNAL 5i CONTROL FIG-.10.

5:} S'GNAL CONTROL [IIIZJQIIJFIIEIFIZI INVENTOR.

5 SIGNAL BY 0.s. FIELD 56 CONTROL HIS ATTORNEY United States TRACK CIRCUIT FOR RAILROAD Oscar S. Field, Rochester, N. Y., assignor to General Railway Signal Company, Rochester, N. Y.

Application November 9, 1953, Serial No. 390,791 2 Claims. (Cl. 246-41) This invention relates to a track circuit for railroads, and more particularly pertains to a track circuit constructed on the closed circuit principle for positively detecting the presence of trains independently of their ability to shunt the track rails.

In the conventional direct current and alternating current track circuits, a source of energy is located at one end flow from the track en-' potential to a value which causes the atnt O Due to varying ballast conditions as between wet and i dry weather conditions, it is diflicult to maintain the interrail potential at a proper value to keep the track relay picked up and yet be of such a value that the usual train shunt will cause the relay to drop away. There are also; condltions where the shunting effect of a train, either be- I cause it is light weight equipment, or because of the character of forelgn material on the track rails, or for other reasons, does not adequately shunt the two track rails and the track relay fails to drop away. This is a-danger-ZT- ous condition and one that the present invention proposes to eliminate.

Generally speaking, and withoutin any way trying to define the exact nature of the invention, it may be stated that this invention proposes to train as a detecting factor. This is to be accomplished by placing suitable means between one track rail and its supporting ties at relatively frequent intervals which means will change its electrical characteristics when subemploy the weight of the jected to the. weight of even the lightest car. It is pro-f posed to connect these electrical elements in multiple by a circuit including the track rail as one of the parallel paths and including an auxiliary conductor as another of the paths. These two parallel paths are then supplied with a suitable source of energy at one end and with a track relay at the other end.

In one form of the invention, it is proposed that these weight responsive elements shall be in the form of a resilient material which changes its electrical conductivity when subjected to pressure. One such material has been found to be a form of conductive rubber produced with an inclusion of a substantial proportion of carbon particles.

In another form of the invention, it is proposed that resilient material be used to normally maintain an insulating space between two metal contact pieces which space will be reduced to zero so that the metal parts will contact each other when the rail is subjected to the weight of a passing car.

vention is to provide a track circuit constructed on the closed circuit principle which is responsive to the presence of a train by reason of its weight.

It is also contemplated in accordance with the present invention that the weight responsive track circuit proposed in accordance with the present invention be combined with a conventional track circuit organization employing the train-shunt characteristics so that the advantages of both organizations may be obtained.

- Thus, it may be said that another object of the present invention is to provide a track circuit organization constructed on the closed circuit principle which is responsive to the presence of a train both by reason of its weight and by reason of its train-shunt characteristics.

A still further object of the present invention is to provide a track circuit organization which is so responsive to the presence of a train by reason of its weight and trainshunt characteristics that the failure of either results in control on the side of safety.

In addition, it may be said that another object of the present invention is to provide a composite weight responsive and train-shunt type track circuit so constituted as to also provide protection against broken rails. In this same connection, it is proposed to organize the apparatus and circuits in a way to reduce the power losses below those ordinarily encountered in the conventional trainshunt type of track circuit.

Other objects, purposes, and characteristic features of the present invention will appear as the description progresses, referencebeing made to the accompanying drawings solely by way of example and in no way in a limiting sense as to the number of different forms which the invention can assume.

In the drawings:

Fig. 1 is a diagrammatic view of a track rail and the interconnected parts arranged in such a diagrammatic fashion as to illustrate the contemplated operation;

Fig. 2 is an end sectional view to show the relative location of parts in one form of the invention;

Fig. 3 is an end sectional view taken on line 3-3 of Fig. 4 illustrating the location of parts constitutingasec- 0nd form of the invention; 7

Fig. 4 is a detailed top view of Fig.3 to show the contemplated electrical connections;

Fig. 5 is a top view of Fig. 2 with the track rail removed and with certain parts shown in section;

Fig. 6 is a top view of Fig. 3 with the track rail removed and certain parts shown in section;

Fig. 7 is a partial sectional plan view of the contact structure of Fig. 3; g

Fig. 8 is an end sectional view similar to Fig. 2 but illustrating the electrical isolation of the weight responsive elements from the track rail;

Fig. 9 is a diagrammatic view of a section of track having weight responsive circuit elements combined with a train-shunt type track circuit;

Fig. 10 is another diagrammatic view of a track section having the weight responsive elements of the present invention combined in a composite circuit organization which is responsive both to the weight of a passing car and its train-shunt characteristics; and

Fig. 11 is a diagrammatic view of a track section-having a weight responsive element of the present invention combined in a circuit organization employing both of the track rails of'the section to be responsive to the presence of a train both by reason of its weight and by reason' of its train-shunt characteristics but organized to avoid the power losses characteristic of a conventional track circuit.

Referring to the drawings, Fig. 2 shows the rail'5 mounted on a tie 6 with a weight responsive variable resistance structure mounted between the two. This-struts ture includes a special tie plate 7 which has bonded to it a block 8 of conductive rubber. On top of this block 8 of conductive rubber is a metal top plate 9 of suitable material also bondedto the conductive rubber.

The rail then rests on the metal top plate 9 and is held in place by the spikes 18 which pass through enlarged holes in the tie plate 7. It is desired to maintain the rail 5 insulated from the tie plate 7 insofar as any connection made by the spikes. between the tie plate and the rail is concerned. For this reason, the holes in the tie plate are made relatively large, and an insulating bushing 11 is inserted through which the spikes 10 may pass. Such an insulating bushing 11 has been shown in Fig; 5.

Referring to Figs. 2 and 5 it will be noted that the tie plate 7 extends beyond the rail into the spacebetween the two rails so that these extending ends of the tie plate may be conductively interconnected as shown in Fig. 1 by a bus bar 15. This bus bar 15 may be of any suitable material preferably of low conductivity and is tightly connected to the tie plates 7 by suitable bolts 16. This connection could be a weld if so desired.

Since considerable weight and stresses are applied to the tie plates, it is generally desirable to make these of a medium hard steel. The top plate 9 may be of a suitable steel or conducting material, but the purpose of this top plate 9 is to make an intimate contact between the conductive rubber block 8 and the underside of the rail 5. In this way, there is an electrical connection of relatively high resistance between the rail 5 and the tie plate 7. When these weight responsive elements are all connected in multiple by the bus bar 15 and rail 5, as shown in Fig. 1, there is in effect a series of variable resistance multiple connections between the rail 5 and the bus bar 15. This has been indicated in a symbolic fashion in Fig. 1 merely to make the diagrammatic illustration of Fig. 1 functionally apparent, but it is to be understood that there are no additional resistances in Fig. 1 and that the variable resistors 18 shown merely represent the variable resistance rubber blocks 8 of the various weight responsive elements along the rail 5, each weight responsive element being constructed as shown in Fig. 2.

A battery B is shown at the right-hand end of Fig. 1

connected through an adjustable limiting resistor R between the rail 5 and the bus bar 15. At the other end a track relay TR is connected between the bus bar 15 and the rail 5 through an adjustable resistor R2. The track relay TR has contacts, such as contact 20 for controlling suitable signals'as desired.

The limiting resistors R and R2 are suitably adjusted so as to' energize the track relay TR and cause itscontacts to be picked up under normal conditions when the variable resistance units 18 are'at their normal value with no train present. When a train enters and causes an appreciable weight to be applied to the rail 5 so that the respective variable resistor uni-ts 18 are compressed, the net resist- :ance between the rail 5 and the bus bar 15 is reduced so that an added current flows through the rail 5 and bus bar 15 as well as the limiting resistor R. This causes a reduced potential across the track relay TR and resistance R2 sufficient to cause the track relay TR to release. When the weight of the train is removed, the variable resistor units 18 of course restore because of their inherent resiliency and the sum of the resistance between the rail 5 and-the bus bar 15 is increased to a normal value which allows the track relay TR to again be picked up.

These weight responsive elements, as above mentioned, include the sections of conducting rubber 8. This conducting rubber is made in the usual way by using carbon black powder mixed in the rubber, but with a relatively higher proportion of such powder than is ordinarily used in making rubber tires and the like. When these blocks 8 are-originally formed with no weight present they have a relatively high resistance in the order of several thousand hrns; but when a substantial weight isapplied, such as 11i fiS JR 'Q Q dQB w hep esent nve ti n. e

resistance of these blocks is reduced to a relatively low value such as 10 to 20 ohms. However, it has been found that the removal of the heavy weight causing the compression of the blocks allows the restoration of the rubber block, but such restoration is not to its original value. By way of an example, it has been found that such restoration may be in the range of 50 to ohms. Thus, it will be seen that the summated resistance between the rail 5 and the bus bar 15 will be dependent upon the number of multiple units employed. Since such a number will be variable depending upon the length of the track circuit, no eifect is here made to indicate the resulting values involved. These relative resistance values of the blocks 8 have been mentioned merely to give an indication of the general characteristics rather than in any limiting sense. It is of course understood that it improved materials are found which have a higher ratio of change upon the application of pressure, such materials can of course be employed within the scope of this invention.

It should be noted that successive track circuits may be formed without requiring insulated joints in the rail since each track circuit is characterized by having its bus bar 15 terminated for its particular length of section. In this way, there is no problem of inserting insulated joints in the rail for the particular length of track to act as a detecting section. This means that the detecting section can be of any desired length regardless of the lengths of rail employed.

Also, this type of track circuit is independent of changes in ballast conditions and is independent of the character of the train-shunt commonly relied upon. In some cases, the train wheels may have a non-conductive or at least highly resistive scale on their outer surfaces due to a continual pounding of the rails as they travel along especially when brake shoes are not provided or at least not used sufiiciently to remove the scale. In other instances, the oiling of the journal boxes and other conditions may cause an oily dirt covering either along the rails or formed on the wheels, and this prevents the usual shunting of the track rails in the conventional track circuit. The track circuit provided in accordance with the present invention wholly obviates these problems since the detection condition is the presence of the weight of the cars.

Although Figs. 2 and 5 merely show the conductive rubber block 8 as intervening between the two metal plates 7 and 9, it is to be understood that suitable structure may be provided, as desired to more adequately enclose this conductive rubber block 8 so as to prevent its deterioration and to prevent the presence of water, oil and the like from coming in contact with it.

With reference to Fig. 3, it will be noted that a slightly modified structure is employed. In this instance, two metal plates 25 and 26, preferably of steel, are separated by a suitable resilient material such as insulating rubber, either natural or artificialtype of rubber, in the form and shape of a ring or'square to enclose a contact assembly. 'This contact assembly comprises a back contact 27 welded to the metal plate 5; :as well as a front contact 28 welded to the metal plate 26. These contacts 27 and 28 are of course normally open due to the fact that the plates 25 and 26 are separated by reason of the expanded condition of the rubber ring 24.

Referring to Fig. 4, it will be seen that the plate 25 has an extending lug which goes out to the bus bar 15 and is bolted thereto by a suitable bolt 36. The plate 26 also has an extending lug to which is bolted a wire connector 29 that is also bolted or otherwise fastened to the web of the rail 5. In this way, there is assured a low resistance contact between the plate 26 and the rail 5 regardless of the intimacy of the contact between the rail 5 and the metal plate 26. It is assumed that the rubber ring 24 is bonded to both the plates 25 and 26 so as to provide a water-proof hermetically sealed chamber in which the contacting parts are located. However, it is gem-009 to be understood that this matter of enclosing the'contacts may assume various forms.

Also, the extending lug of the plate 25 is connected to the bus bar 15 by bolts 36 to make possible an intimate electrical connection with the bus bar without depending upon the incidental electrical contact between the plate 25 and the tie plate 7. In this form of the invention, it is optional whether or not the tie plate 7 is elongated so as to extend to the bus bar 15. As shown, each tie plate 7 is bolted to the extending lug 25 as well as to the bus bar 15; but this is not necessary. Actually, the tie plates 7 need to be only the standard size, but with enlarged holes for the passage of the spikes 10. These holes are sufiiciently large to permit the use of insulating bushings 11 so as to avoid electrical contact between the rails and tie plate 7.

It is contemplated that these contacting units of Fig. 3 are used in the same general organization shown and described in connection with Fig. 1. In this form of the invention, the normal resistance between the rail 5 and the bus bar 15 will be considerably higher than when the units of the form of Fig. 2 are used, since the rings 24 are contemplated as being made of non-conductive rubber or like insulating material and the contacts 27 and 28 are normally open. However, regardless of this normally high resistance (low conductance), when the presence of a train is effective to subject this portion of the rail 5 to a substantial weight, the contacts 27 and 28 of the adjacent units make contact and the resistance between the rail 5 and the bus bar 15 is reduced to a relatively low value. The operation is then the same as previously described in connection with Fig. 1 where the potential drop across the track relay TR and resistance R2 is reduced to such a value as to cause the relay TR to drop away. When the weight of the train is removed, the resiliency of the rubber rings 24 cause the plates 25 and 26 to separate which in turn separates the contacts 27 and 28. This breaks the connection between the bus bar 15 and rail 5. When the train has wholly removed from the track section and all of these contact units are restored, obviously the resistance between the parallel connections is raised to a relatively high value. This causes the potential between the rail 5 and bus bar 15 to be restored to a value which Will again pick up the track relay TR.

Although the modified form of Fig. 3 is considered to be used in the same general organization of Fig. 1, this organization by showing the resistor units 18 might more accurately give the concept if they were replaced by a showing of contacts. However, even with this Fig. 3 form of the present invention there will be some small leakage between the rail 5 and the bus bar 15 even though the contacts 27 and 28 are separated in all of the units. This is due to the presence of dirt, moisture or the like.

When the presence of a train causes some of these contacts to be closed, the resistance in the circuits which they close is of course not zero, but is of some small finite value which is in multiple with any leakage resistances. This makes a substantial reduction in the net resistance of the multiple paths. Thus, even though contacts are employed in this Fig. 3 form of the invention, the symbolical representation of variable resistance units 18 in Fig. 1 does in fact represent the functional characteristics of this organization.

The above description has more particular reference to a track circuit organization responsive to the presence of a train by reason of its weight alone. However, it is to be understood that the present invention also contemplates the combining of weight responsive elements in connection with a track circuit responsive to train-shunts in such a way as to have a composite control so that the failure of either characteristic is on the side of safety. Such a composite track circuit is desirable so that broken rail protection will be provided in connection with a Weight responsive track circuit.

g This combined track circuit organization has been illustrated in Fig. 9 where the rails 50 of a suitable track section have a track relay T connected at one end across the rails through a variable resistor 51. At the other end of this track section, a track battery 52 is connected through the limiting resistor 53 across the track rails 50. The track relay T is normally energized and is released when a train enters the track section and provides the usul shunt between the two rails 50 of the section. It should be understood that these rails 50 constituting the track section must be insulated from the adjoining rails of the sections at opposite ends thereof.

Weight responsive elements 18 are located beneath one of the rails 50 at spaced intervals, such as for the several ties supporting therail, and these elements 18 are then connected in multiple across the two bus connections 54 and 55. These weight responsive elements 18 are the same as the element 8 shown in Fig. 2. It is to be understood, however, that the weight responsive contact means of Fig. 3 could as well be used. If the form of Fig. 2 is employed, electrical connections from the plates 7 and 9 are respectively made to the buses 54 and 55. If the form of Fig. 3 is employed, electrical connections are made from the plates 25 and 26 respectively to the buses 54 and 55.

The two buses 54 and 55 have the track relay TR connected across them through the variable resistor R2 the same as shown in Fig. 1. Similarly, the track battery B and the limiting resistor R are connected across the buses 54 and 55 at the other end to complete the weight responsive track circuit corresponding to the arrangement described in connection with Fig. 1. In this form of Fig. 9 the bus 54 is employed as a conductor in place of the rail 5; while the bus 55 is employed the same as the bus 15.

It should also be understood in this connection, that the buses 54 and 55 have been indicated as being bus bars or strips of metal to which the various connections are made; but it is to be understood that these buses 54 and 55 may just as well be a cable which may be buried along the trackway and merely have external connections for each of the weight responsive elements 18, for the track relay TR and track battery connections. Regardless of the particular manner of providing the multiple electrical paths, the operation is exactly the same as described in connection with Fig. 1..

Under normal conditions, both track relays T and TR are picked up closing their respective contacts 56 and 57.

These contacts may be used in any desired way for the control of trafiic over the railroad, but in the drawings it has been indicated that they jointly provide suitable signal control. The presence of a train in the track section shunts the two track rails 50 and causes the track relay T to drop away. Likewise, the weight of the train acts upon the weight responsive elements 18 to cause the release of the track relay TR. Since the track relay T receives energy through the track rails 50 by reason of their providing a complete circuit, the fissure of either rail at any point will cause the release of this track relay T. With the contacts 56 and 57 connected in series in the circuits to be controlled, the presence of a train will be assured of being indicated either by reason of its train shunt or by reason of its weight characteristics or by both; and also the fissure of either track rail will be indicated by the release of the track relay T. In this way, the signal control is assured of being on the side of safety in the event of any kind of failure. It is obvious that the train must wholly vacate the section before both track relays T and TR can be restored to normal energized conditions.

Referring to Fig. 10, the rails 50 constitute the two rails of a single track section suitably insulated from the adjoining rails at the opposite ends, the same as described in connection with Fig. 9. A bus counection/ elements 18.

1 5 is shownthe same as in Fig. 1 connected to a plurality of weight responsive elements 18. These weight responsive elements18are assumed to be either of the type shown in Fig. 2 or the type shown in Fig. 3. In either case, the upper plate of the weight responsive element is an electrical contact with or positively connected to the associated track rail to provide an intimate electrical connection.

The bus 15 is permanently connected through wire 57 to the upper rail 50. In this way, the normal resistance of the weight responsive elements 18 in multiple is connected between the lower rail 50 and the upper rail 50. This resistance is, of course, in multiple with the ballast leakage resistance between the two rails.

The variable resistor 51 and the limiting resistor 53 are adjusted so that the track relay T is normally energized with no train present; but when a train enters the track section its shunting characteristics are effective to shunt the two rails 50 and increase the current flow through the limiting resistor 53 and thus reduce the interrail potential for the track relay T. In addition, the weight of the train causes the weight responsive elements 18 to reduce their resistance, in a manner previously explained, which is also connected between the two rails 50 and adds to the shunting effect of the train. In other words, both the train-shunt and the weight characteristics are effective to reduce the potential across the relay T to a value that causes it to be released. When the train entirely leaves the section, the normal conditions are restored and the track relay T is again picked It will be noted that the normal energizing current ")r the track relay T flows through both track rails 50 so that the fissure of either rail reduces the current flow orinterrupts it entirely causing the track relay T to drop away. In this way, broken rail protection, which is a desirable feature, is adequately provided. Since the Weight responsive elements 18 are in multiple, they produce a low resistance path between the upper rail and the lower rail, which due to wire 57 is effective at the relay end of the track circuit. This is a desirable characteristic since a shunt at the relay end is more effective to drop the. relay.

It is tobe understood that this shunting effect produced by the weight responsive elements is sufficient to cause the release of the track relay T even though the train shunt is ineffective and similarly the presence of a proper train-shunt is sufficient to cause the release of the track relay. T even though the weight responsive elements are ineffective, as might occur if the wire 57 were broken. 7 In brief, the track relay T is responsive to either or both of the controlling conditions, i. e. a train-shunt or a weight responsive shunt.

Referring to Fig. 11, the rails 50 constitute a single track section suitably insulated from the adjoining rails at the opposcite ends, the same as described in connection with'Figs. 9 .and 10 above. Two bus connections 54 and 55 are shown the same as in Fig. 9, and these two buses are connected to a plurality of. weight responsive In this form of the invention, these weight responsive elements are assumed to be of the type shown in Fig. 8 which is the form of Fig. 2 but with an insulating strip 59 separating the plate' 9 from the track rail. This is so that the connections between the plates 7 and 9 and the buses 54 and 55 respectively will not in any way be connected to thc lower track rail 50. In this connection, it should also be understood that the form of the invention shownin Fig. 3 may be employed, if a similar insulating strip 59 is used to insulate theplate 2.6 from the track rail 50. Of course, the connections 29 between the plate 26 and the rail would be omitted in this case (seeFig. 3).

It is noted that the left-hand ends of the two track rails- 50in Fig. ll are connected together by wire 58.

L 8 Also, the right-hand end of bus v54 is connected through Wire 60 to the lower rail 50. The track relay T is connected through the variableresistor -51 across the buses 54. and 55 at their left-hand ends. The battery 52 and limiting resistor 53 are connected between the right-hand end of lowerbus 55 and the right-hand end of upper rail 50. With these connections, the normal path for the energizing current of the track relay T includes both .railsin series with both buses 54 and 55. Any break in a rail, or a bus will cause the track relay T to release. Also, any breakage or interruption of the wires 58 and 60 would likewise cause the track relay T to drop away.

When a train enters the track section, its weight is effective on the weight responsive elements 18 to cause the buses 54 and 55 to be effectively shunted to cause the release of the track relay T. The train-shunt effect across the rails is relatively unimportant. Obviously, it changes the circuit the least when the train first enters and increases as the train travels to the right when the rail resistance is shunted out of the circuit; but since this rail resistance is a relatively low value such as in the order of .06 ohm per thousand feet of single rail, the current in the circuit is not substantially increased by reason of the train-shunt.

It will also be noted that the potential between the two rails 50 is relatively low since it is only the potential drop of the current in the rails, and for this reason, there is practically no power loss through the ballast resistance as is found in the usual track circuits. In other words, the rails 50 are included in the track circuit for the purpose of obtaining broken rail protection, but this inclusion does not cause the usual power losses because of the relatively low potential between the two rails.

The buses 54 and may be of any suitable type, but it is assumed that they are suitably insulated from each other and from adjacent grounds. These buses may be in an armored cable and buried, but with such cable having external connections for the respective weight responsive elements 18, or they may take any other form suitable for practical conditions. However, it is assumed that they are sufficiently electrically isolated from the ground and from each other that the losses ordinarily encountered due to ballast conditions and the like are practically eliminated.

In this form of the invention of Fig. 11, it should be observed that the train-shunt does not act upon the track circuit to effect release of the track relay T, so that it is unimportant as to what particular values the train-shunt may have. In other Words, the operative conditions for the track circuit is the weight responsive elements, while the inclusion of the rails provides the desired broken rail protection, and the organization is operative regardless of any train-shunt condition.

Several forms of circuit connections for track circuits contemplated in accordance with the invention have thus beenshown and described; but it is to be understood that the selection of a particular one should be in accordance withthe practical requirements of each installation.

Having described a weight responsive type of track circuit with a plurality of forms of different units to embody 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 depart ing from the spirit or scope of the present invention.

What I claim is:

l. A train presence detecting organization for railroads comprising, a plurality of resilient weight responsive elements mounted at spaced intervals beneath at least one of the rails of a section of trackway each of said elements characteristically acting to reduce its electrical resistance from a normal value when compressed, a track relay at one end of said track section, a source of energy and a limiting resistor at the other end of the track section, conductive circuit means extending through said section of track for connecting said Weight responsive elements in multiple with each other and With said track relay at said one end of said track section, other circuit means for connecting said source of energy in series with said limiting resistor to said multiple connection to supply said track relay with normal energizing current, said Weight responsive elements acting when said track section is occupied by a train to reduce the resistance across said multiple connection to draw added current through said series limting resistor from said source of energy to reduce the resulting potential across said multiple connection and cause the release of said track relay.

2. In a train presence detecting organization for railroads, a plurality of carbonized-rubber variable resistor elements each having a normal resistance and each effectively reducing its resistance when compressed under pressure, said elements being mounted to support at least one rail of the section of trackway, circuit means including said one rail and a separate conductor extending through said section for completing a parallel circuit for said plurality of elements, a source of energy connected in series With a limiting resistor across said parallel circuit at one end of said section, a track relay connected across said parallel circuit at the other end of said section, said limiting resistor being adjusted to allow said source of energy to normally actuate said track relay, but eifective, when the resistance of said parallel circuit is etfectively reduced by reason of the presence of a train causing said elements to reduce their resistance, to cause a sufiicient potential drop to effect the release of said track relay.

References Cited in the tile of this patent UNITED STATES PATENTS 184,842 Conklin Nov. 28, 1876 1,233,231 Hawley July 10, 1917 1,491,295 Fink Apr. 22, 1924 1,577,981 Otto Mar. 23, 1926 1,610,800 Savarack Dec. 14, 1926 1,662,444 Tegeler Mar. 13, 1928 1,814,465 Becq July 14, 1931 2,042,606 Kotowski June 2, 1936 2,105,930 Reichard Ian. 18, 1938 2,108,759 Turman Feb. 15, 1938

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