|Publication number||US3450874 A|
|Publication date||Jun 17, 1969|
|Filing date||Mar 31, 1967|
|Priority date||Mar 31, 1967|
|Publication number||US 3450874 A, US 3450874A, US-A-3450874, US3450874 A, US3450874A|
|Inventors||Whitten James R|
|Original Assignee||Gen Electric|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (5), Referenced by (12), Classifications (7)|
|External Links: USPTO, USPTO Assignment, Espacenet|
June 17, 1969 J. R. WHITTEN 3,450,874
RAILWAY'TRACK CIRCUIT Filed March 31, 1967' INVENTOR. J'ANES R. WHITTEN HIS ATTORNEY June 17, 1969 J.. R. WHITTEN 3,450,874
RAILWAY TRACK CIRCUIT Filed March 31, 1967 Sheet 3 0f 3 DETECTOR F' G 4 JAMES R.
HIS ATTORNEY United States Patent US. Cl. 246-34 17 Claims ABSTRACT OF THE DISCLOSURE A track circuit for a section of railway track wherein a source of A-C signal energy is coupled to one end of the section, a receiver is coupled to the other end thereof to receive the signal energy from the source and a number of inductances are connected across the track rails in spaced relation along the section. Either in addition to or instead of the spaced inductances, at least one feedback means is providedpositive feed-back at the receiver, negative feed-back at the source or both of such feed-back arrangements; the feed-back signal being derived from the track circuit and proportional to the signal energy therein.
The present invention relates to rail vehicle detection systems and more particularly relates to a track circuit of the alternating current type for a section of railway track.
In the automatic control of rail or track vehicles such as trains, subways, rapid transits and the like, it has been necessary to control the travel of the vehicle relative to the position of another vehicle travelling in advance thereof and on the same track to avoid sudden stoppings or actual collision between the vehicles. One common method of detecting the location of the advance rail vehicle is to divide the right of way into a plurality of zones with the rails disposed wtihin such zones forming a part of a zone electrical circuit. A transmitter transmits a signal along the zone and a detector detects the level of the signal. The presence of a train is detected by a drop in the signal level when a train enters a zone due to the shunting effects of the train wheel and axle assembly. A difiiculty with the zone detection method of detecting train presence as used in the prior art has been that rain, snow, ice, or the like cause changes in the ballast impedance which also causes a change in signal level due to changes in shunting and leakage effects. As a result, it was diflicult to determine whether the drop in signal level received by the detector was due to the shunting elfect of the wheels and axle of the train within the zone or due to a drop in level produced by changes in ballast impedance due to weather conditions.
Accordingly, an object of the present invention is to produce a system of rail vehicle detection wherein the change in signal level due to changes in weather is minimized.
A further object is to produce a system of train detection wherein compensation is provided for the effect of variations in ballast impedance due to weather.
A further object is to produce a system which is substantially independent of weather variations.
Briefly stated, in accordance with one aspect of this invention, a new and improved track circuit for a section of railway track comprises aplurality of inductance means connected across the track rails of the section, a trans-.
mitter of alternating current signal energy coupled to the track rails and a receiver coupled to the track rails for receiving the signal energy from the transmitter. In the track circuit, the inductance means are operative to improve the ability of the receiver to distinguish the shunting effect across the track rails due to variation in ballast impedance due to weather and that of the wheels and axle assembly of a vehicle present on the track section.
In another aspect of the invention further improvement may be obtained by employing positive feed-back means at the receiver or negative feed-back means at the transmitter; still greater improvement being achieved by a combination of positive feed-back means at the re ceiver and negative feed-back means at the transmitter and the inductance means wherein the inductance means coact with such feed-back means.
Referring now to the drawings:
FIG. 1 is a fragmentary schematic view of a track occupancy zone in accordance with the present invention;
FIG. 2 is a fragmentary schematic view of a track occupancy detection zone in accordance with another embodiment of the invention and wherein feed-back arrangements are provided to the track occupancy zone shown in FIG. 1 at both ends of the zone;
FIG. 3 is a fragmentary schematic view illustrating the. transmitter negative feed-back arrangement; and
FIG. 4 is a fragmentary schematic view illustrating the detector positive feed-back arrangement.
In FIG. 1 there is shown occupancy zone or block including a source of alternating current signal energy, shown as transmitter means 16, coupled at one end of the zone and a receiver 18 coupled at the other end thereof for receiving the signal energy from the source. In order to increase the length of the zone while still achieving reliable occupancy detection in spite of varying ballast conditions, a plurality of inductances 14 are connected across the track rails in spaced apart relationship along the zone. The limits of the zone are defined in any suitable manner, shown as low impedance cross connectors 10 and 12. Alternatively, resonant circuits, series resonant at the frequency of the signal energy may be employed for this purpose. Also, if desired, insulated joints may be employed for this purpose as is Well known.
As shown in FIG. 1, a transmitter 16 of alternating current signal energy is coupled in any suitable manner to the train occupancy zone circuit such as by the inductive coupling means 24 disposed between the track rails 4 and 6, to apply A-C signal energy to the zone. A receiving means 18 may be similarly inductively coupled to the circuit or may be connected, as shown, directly across the rails 4 and 6 by suitable conductors. Inductances 14a, 1411, 14m and 1411 are connected across the rails 4 and 6 at spaced intervals throughout the zone as shown. Preferably, the inductances 14 are of a combined shunting impedance which is less than the resistive shunting effect of the worst weather conditions, but greater than the resistive shunting eifect of the wheels and axle of a train to be detected.
The use of the inductances functions to increase the sensitivity of the receiver to the presence of a train within the zone especially in regard to the shunting effects due to varying ballast conditions (for example, as between wet or dry weather conditions) on the ability of the receiver to differentiate between a change in inter-rail potential due to verying ballast conditions and a change in inter-rail potential due to the actual presence of a train in the zone. Since the detection sensitivity is increased, the use of spaced inductances along the zone makes possible an increase in the zone length in which reliable detection may be accomplished.
The value of the inductances, the spacing and the thereof, and the frequency of the signal energy are all factors which must be considered since they determine the zone length, detection sensitivity, worst case weather detection and worst case train resistance detection. In a particular operable arrangement, reliable detection was achieved in a zone 2,500 feet long with as few as 3 two inductances equally spaced therein each of which had an inductance of 0.3 millihenry. In the foregoing particular arrangement, the minimum weather shunting resistance was taken as 10 ohms per 100 feet of track.
Referring to the drawings, FIG. 2 illustrates a preferred embodiment of the invention wherein, in addition to the spaced inductances, there is also provided negative feed-back at the transmitter and positive feed-back at the receiver. It is to be understood that the use of feed-back only (either at transimtter or receiver or both) without the use of spaced inductances provides for increased detection sensitivity over previously known arrangements.
As shown in FIG. 2, the right of way is divided into a plurality of identical zones in a manner similar to that shown in 1 16. 1 wherein each zone is defined by rail sections 4 and 6 and shorting means 10 and 12 which together establish an electrical circuit. A plurality of spaced inductances such as at 14a, 14h, 14m and 1411 are connected across the rails at spaced intervals throughout the zone. Again inductors 14 are of a combined shunting impedance which is preferably less than the resistive shunting effect of the worst weather conditions, but greater than the resistive shunting effect of the wheels and axle of any train which is to be detected. A transmitter means 16, including a negative feed-back arrangement (FIG. 3) is operably connected to the zone circuit to apply alternating current signal energy thereto. A receiver means 18, including a positive feed-back arrangement (FIG. 4), is operably connected to the zone circuit so as to detect the level of the signal energy received by it from the transmitter 16 and thereby detect the presence of a train Within the zone. The inductances function in the manner previously described in connection with FIG. 1.
In the embodiment of FG. 2, transmitting means 16 includes a negative feed-back arrangement whereby the current in the track section tends to remain at a constant level so that the signal energy transmitted in the zone is independent of variations in ballast impedance caused by changes in weather conditions. One arrangement for achieving such negative feed-back together with associated fail-safe features is shown in FIG. 3. As shown, an oscillator generates a continuous wave signal which is fed by conductors 17 and 19 to a summer 20 where the signal is combined with the feed-back signal from the feedback conductors 33 and 35. The output of summer 20 is fed by conductors 21 and 23 to an amplifier 22 where the combined signal is amplified. The amplified combined signal is then coupled to the zone circuit by a suitable means, such as by way of conductors 25 and 27 to inductive coupling means 24 where the signal is inductively coupled to the rails.
Conductors 26 and 28 are connected to one of the rails at a spaced interval to derive a signal which is proportional to track current. This arrangement provides the voltage drop across the rail portion 30 between the conductors 26 and 28. In order to avoid the pick up of stray or extraneous signals, the conductors 26 and 28 may be shielded as indicated schematically at 3-4. The conductors 26 and 28 connect the proportional to track current signal to a phase shifter 36 where the phase is shifted so as to be of a different phase than the original signal in the track rails. The output of phase shifter 36 is fed by conductors 37 and 39 to an amplifier 38. The output of amplifier 38 feeds by conductors 33 and to the summer 20 where the feed-back signal is combined with the original signal from oscillator 15 and fed to the zone circuit by coupling means 24. Thus, the feedback is current feed-back which is operative to tend to hold the transmitter output current constant.
Since the resistance shunting effect caused by lowered ballast impedance due to adverse weather conditions such as snow, rain, sleet and the like, tends to increase the track current, the negative feed-back arrangement is used to hold this current constant. By this means a transmitted signal will be of a more constant level resulting in a higher effective source impedance and improved detection of trains. Thus, where there is a drop in signal level being detected by the receiver means, the drop will be greater due to the presence of a train than to changes in weather due to the differences in shunting impedance.
The phase shifting means 36 may be any standard phase shifting network which provides a negative feedback when the load across the track circuit is inductive. By such phase shifting arrangement the voltage picked up across conductors 26 and 28 is shifted in phase by an amount sufficient to cause the feed-back current to be out of phase with the original current so as to maintain constant current at the transmitter. Thus as the current at the transmitter tends to increase as the shunting resistance due to weather decreases, the voltage feedback to the amplifier operates to lower the the output of the amplifier proportionally. As a result, more constant current at the transmitter is achieved.
In order that the system be fail-safe with respect to a malfunction of the transmitting means, a fault indicating means 40 is provided to send an output along conductors 51 and 52 to turn off the oscillator 15 when the fault indicating means 40 receives an input signal from any one of several checking circuits. The fault indicating means may be a static or dynamic switching deivce which provides a single output upon receipt of any of several input signals.
The first input signal is from a first checking circuit which is designed to guard against malfunction of the amplifier 22. Should the amplifier 22 independently oscillate due to some defect, its output would become independent of the feed-back and a signal frequency shift may occur. In order to shut down the system should this occur, a checking circuit consisting of conductors 41 and 42, summer 43, conductors 44 and 45, amplifier 46 and conductors 47 and 48 is connected across the amplifier 22. The amplifier 6 is selected so as to have the same gain as amplifier 22. Summer 3 provides a difference between the output of amplifier 46 and the output of amplifier 22. Where the aforesaid amplifier outputs are equal, the summer output which feeds along conductors 49 and 50 to the fault indicating means 40 is zero. Should the output of amplifier 22 be different from the output of amplifier 46, an output would appear at summer 43 and be transmitted by conductors 49 and 50 to the fault indicating means 40 thereby causing an output at the fault indicating means which is fed by conductors 51 and 52 to turn off the oscillator 15 by a suitable switch (not shown) located in the oscillator.
A second checking circuit is connected across amplifier 38 and operates in the same manner as was described for the first checking circuit for amplifier 22. The second checking circuit consists of conductors 53, 54, amplifier 55, conductors 56 and 57, summer 58 and conductors 59 and 60. Amplifier is of the same gain as amplifier 38 and thus the output at summer 58 is zero when the amplifier 38 is functioning properly. Should amplifier 38 oscillate or otherwise fail, there will be an output at summer 58 which is fed by conductors 61 and 62 to the fault indicator 40, the output of which will be fed by conductors 51 and 52 to the oscillator 15 where the switch (not shown) shuts off the oscillator.
A third checking circuit checks for a fault in the feedback loop. For this purpose a summer 64 is connected by conductors and 65, and 59 and 66 across the output of amplifier 38 which is in the feedback loop. A reference voltage from a reference voltage source 68 is fed into the summer 64 by conductors 69 and 70. The reference voltage from source 68 should be established so as to have the same frequency, amplitude and phase as the normal output of amplifier 38. These values may be determined by analyzing the output of amplifier 38 when it is operating properly. The reference volt-age may be conveniently produced by feeding a portiton of the output of the oscillator by conductors 71 and 72 to the voltage source 68 wherein the proper phase and magnitude is obtained by suitable transformer and phase shifting means (not shown). By feeding both the reference voltage and the output of amplifier 38 to summer 64 any change in the feedback loop will cause an output at summer 64 which feeds by conductors 74 and 75 into the fault indicator 40 and thence by conductors 51 and 52 to a switch (not shown) in the oscillator 15 to turn it off.
In accordance with a further aspect of this invention, receiver means 18 includes a positive feedback arrangement (FIG. 4) whereby the difference in signal level between train and no train conditions is made to be much greater than it would be without the feed-back and whereby such feed-back is upset or destroyed by the presence of a train, thereby enhancing the ability to distinguish between changes in signal level due to variations in ballast impedance caused by adverse weather conditions and changes in signal level due to the presence of a train.
For example, the presence of a train within the zone is detected by the difference in the level of the signal energy from the transmitter at receiver means 18. To this end, receiver means 18 may include a suitable detector operative to indicate the presence or absence of a train within the zone by the difference in the level of signal energy in the track circuit. The detector may be, for example, a threshold detector known in the art whereby the output of the detector indicates the presence of a train when the signal detected across the track rails is below the preselected threshold. However, the level of the signal detected across the track rails may also be lowered by the shunting resistive effect of changing ballast impedance due to changing weather conditions, thereby making it difficult to distinguish between changes caused by a train and changes caused by weather.
Accordingly, it is desirable to produce a signal whereby differences in signal change due to shunting by weather and shunting by a train are maximum. The presence of the inductances and/ or the positive feed-back at the receiver means 18 provide such difference whereby the shunting effect due to weather has a much lesser effect than a train on signal level.
One arrangement for achieving the foregoing positive feed-back at the receiver is shown in detail in FIG. 4. As shown, a suitable detector 80 is coupled to the track r-a-ils 4 and 6 for detecting the change in signal energy therein from the transmitter means 16 when a train is present within the track section. Conveniently, detector 80 may be connected across the rails 4 and 6 by the conductors 81 and 82 of a suitable coaxial line 83 which provides shielding from stray or extraneous signals.
A feed-back signal proportional to the signal energy in the track circuit at the receiver is obtained and suitably shifted in phase so as to be in phase with the voltage at the receiver when the track circuit shunting impedance is predominantly inductive-which is the case when there is no train present within the track section.
To this end, conductors 85 and 86 of a suitable coaxial line 89 are connected in spaced apart relation along rail 6. The voltage across the rail portiton 87 between condnctors 85 and 86, therefore, is proportional to the current in the rail. This voltage signal is then fed to a phase shifting network 88 and thence over conductors 91 and 92 to an amplifier 93.
The output of amplifier 93 is applied over conductors 94 and 95 to drive a coupling loop 98 which is disposed in inductive relationship with the track rails 4 and 6 so that a voltage proportional to the current in the rail and having the same phase angle is induced therein; the proper phase angle having been provided by phase shifting network 88.
Preferably, coupling loop 98 is disposed as shown between the zone terminating shorting bar 12 and the detector connections 81 and 82. The voltage feed-back signal is thus applied to the feed-back circuit 102 composed of rail portion 100, inductance 14n, rail portion 103 and shorting bar 12.
It is to be understood that a portion of the feed-back signal would also appear in the circuits which include all of the inductances such as 14a, 14b and 14m up to the other shorting bar 10. However, the first inductance 14a is the most significant inductance and establishes the effective circuit for the feed-back purposes.
Since the voltage induced in the track feed-back circuit 102 is in phase with the original voltage resulting from the signal current from the transmitting means 16, it is positive feed-back thereby increasing the magnitude of the voltage detected across conductors 81 and 82. The phase shift network 88, therefore, is made to have the right amount of phase shift so that with an inductance 14n across the tracks, the original current and the induced voltage will constitute positive feed-back.
The induced voltage is fixed by the gain of the amplifier 93 to be a constant, complex multiplier of the signal current from the transmitting means 16. The signal current is constant due to the series impedance of the track section between transmitter 16 and detector 80. The impedance of rail portion is effectively increased by the positive feed-back to increase the sensitivity of the detector 80 to the presence of a train. Thus, the voltage at the input terminals of detector 80 is the sum of the voltage drop due to the impedance of the rails and the induced voltage. By suitable selection of the gain, the impedance at the detector is increased, thereby increasing the detection sensitivity.
It is to be noted that inductance 14m and the other inductances were selected to be of an impedance value preferably less than the impedance value of the resistive shunting effect due to weather; thus variations in the shunting effect due to such weather will have little effect on the feed-back since the impedance of the inductances is less than the lowest impedance of weather shunting effects. However, when a train appears in the zone, the wheels and axle assembly of the train, having a resistance of between zero and two ohms, shunts the inductance and destroys the necessary phase relationships to achieve the positive feed-back thereby causing a substantial drop in the voltage across points 81 and 82. Thus, the present system, rather than merely detecting the difference in signal level between the shunting effect of adverse weather and the shunting effect of the wheels and axles of a train, detects the presence of a train by destroying feedback thereby resulting in a more substantial signal drop which is independent of weather.
In order to provide a fail-safe feature, a checking circuit is connected across the input and output of amplifier 93 whereby a check is provided against oscillation of the amplifier 93. The checking circuit consists of condnctors and 111, amplifier 112, conductors 113 and 114, summer 115 and conductors 116 and 117. As was the case with the transmitter checking arrangement, the amplifier 112 has the same gain as the amplifier 93. Under ordinary operating conditions the output of summer 115 will be zero. However, should the amplifier 93 oscillate or otherwise malfunction and thereby become independent of the feedback relationship, an output will appear at summer 115 and be fed by conductors 119 and 120 to a suitable switch (not shown) in the detector to shut off the detector, thereby resulting in a zero detector output.
It is to be understood that the information concerning the presence or absence of a train within a zone can be relayed to a succeeding train if desired by suitable means known in the art.
While preferred results are achieved by utilizing the combination of negative feed-back at transmitter 16, positive feed-back at detector 18 and a plurality of inductance means disposed across the rails, it is to be understood that significantly improved results over the prior art may be obtained by utilizing either of the foregoing features only. That is, either negative feed-back at the transmitter, the positive feed-back at the detector or the plurality of inductance means disposed across the rails, or any combination thereof.
Having shown preferred embodiments of an improved train detection system, it is believed obvious that other modifications and variations of the invention are possible in the light of the above teachings.
\ What I claim as new and desire to secure by Letters Patent of the United States is:
1. A track circuit for a section of railway track comprising:
(a) a transmitter of signal energy coupled to said section of railway for applying signal energy thereto;
(b) a receiver coupled to said section of railway track for receiving the signal energy from said transmitters; and
(c) a plurality of inductance means connected across the track rails in spaced relation along said section of railway track between said transmitter and said receiver operative to increase the change in the level of the signal energy at said receiver between the conditions when no vehicle is present within the section of railway track and the condition when a vehicle is present therein.
2. The track circuit of claim 1 including at least one feed-back means operably coupled to said track circuit for deriving a signal from said track circuit, shifting the phase thereof and feeding the phase shifted signal so produced back to said track circuit.
3. The track circuit of claim 1 wherein said transmitter is provided with means to produce a negative feedback signal proportional to the signal energy in said track circuit and means applying said feed-back signal to said transmitter so that the signal energy in said track circuit tends to be maintained at a constant level.
4. The track circuit of claim 1 wherein said receiver includes means for producing a positive feed-back signal proportional to the signal energy in said track circuit and means applying said positive feed-back signal to said receiver.
5. The track circuit of claim 3 wherein said receiver includes means for producing a positive feed-back signal proportional to the signal energy in said track circuit and means applying said positive feed-back signal to said receiver.
6. A system for detecting the presence of a rail vehicle according to claim 2 wherein the impedance of said inductance means is greater than the resistive impedance of the wheel and axle assembly of any train which is to be detected.
7. A system for detecting the presence of a rail vehicle according to claim 6 wherein said inductance means exhibits less shunting impedance than the minimum circuit shunting resistance which may be produced by variations in track ambient conditions due to weather conditions.
8. A system for detecting the presence of a rail ve hicle according to claim 2 wherein said feed-back means is a negative feed-back means associated with said rails adjacent said transmission means, whereby a portion of said signal is picked up from the rails and fed back to the transmission means with the phase of the feed-back signal shifted so as to reduce variations in the current output of the transmission means.
9. A system for detecting the presence of a rail vehicle according to claim 8 wherein the phase and amount of the feed-back signal is adjusted as the shunting resistance between the rails decreases so as to maintain the current output of the transmission means substantially constant as the shunting impedance of the rail circuit varies due to variations in track ambient conditions.
10. A system for detecting the presence of a. rail vehicle according to claim 2 wherein said feed-back means is a positive feedback means associated with said rails whereby a portion of said signal is picked up from the rails and fed back to the rails with the phase of the feedback signal shifted so as to increase the signal input to the receiver means.
11. A system for detecting the presence of a rail vehicle according to claim 10 whereby the phase of the feed-back signal is determined by the impedance of said inductance means when a train is absent from the circuit and by the resistance of the train wheel axle assembly when a train is present in the circuit whereby positive feedback is established by the absence of a train and destroyed by the presence of a train.
12. A system for detecting the presence of a rail vehicle comprising: a pair of rails; means to establish a circuit which includes said rails; transmission means operably connected to said circuit for transmitting a signal in said circuit; receiver means operably connected to said circuit for detecting said signal; negative feed-back means associated with said circuit adjacent said transmission means whereby a portion of said signal is picked up from the circuit and fed back to the circuit with the phase of such feed-back signal shifted so as to reduce variation in the current output at the transmission means.
13. A system for detecting the presence of a rail vchicle comprising: a pair of rails; means to establish a circuit which includes said rails; transmission means operably connected to said circuit for transmitting a signal in said circuit; receiver means operably connected to said circuit for detecting said signal; positive feed-back means associated with said circuit adjacent said receiver means whereby a portion of said signal is picked up from the circuit and fed back to the circuit with the phase of such signal shifted so as to be the same as the original signal to increase the voltage level of the signal at the receiver means.
14. A system for detecting the presence of a rail vehicle according to claim 13, wherein a train travelling on the rails within the circuit provides a resistive shunting impedance which shifts the phase of the feed-back signal thereby destroying the positive feed-back so as to cause a decrease in the signal level at the receiver means input.
15. The track circuit of claim 1 wherein the impedance of said inductance means is greater than the resistive impedance of the wheel and axle assembly of the train to be detected and less than the minimum circuit shunting resistance which may be produced due to variations in track ambient conditions.
16. The track circuit of claim 15 including at least one feed-back means operably coupled to said track circuit for deriving a signal therefrom, shifting the phase of said signal and feeding the phase-shifted signal so produced back to said track circuit.
17. The track circuit of claim 5 wherein the impedance of said inductance means is greater than the resistive impedance of the wheel and axle assembly of the train to be detected and less than the minimum circuit shunting resistance which may be produced due to variations in track ambient conditions.
References Cited UNITED STATES PATENTS 2,558,445 6/ 1951 Laurenson. 2,985,752 5/1961 Fines et al. 3,011,050 11/1961 Staples. 3,035,167 5/1962 Luft. 3,046,392 7/ 1962 Luft.
ARTHUR L. LA POINT, Primary Examiner.
US. Cl. X.R. 246-422,
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|Citing Patent||Filing date||Publication date||Applicant||Title|
|US3610920 *||Dec 4, 1969||Oct 5, 1971||Gen Signal Corp||Apparatus and method for deriving a uniform time warning|
|US3854683 *||Dec 10, 1973||Dec 17, 1974||Wabco Westinghouse||Detector track circuit|
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|US4166259 *||Nov 16, 1977||Aug 28, 1979||Messerschmitt-Bolkow-Blohm Gmbh||Flat high frequency cable|
|US4420133 *||Jul 2, 1979||Dec 13, 1983||Jeumont-Schneider||Device for the transmission of information through the rails between a railway track and a group of vehicles running along this track|
|US4641803 *||Jan 23, 1985||Feb 10, 1987||Westinghouse Brake And Signal Company Limited||Track circuit|
|US4665831 *||Mar 5, 1985||May 19, 1987||The Furukawa Electric Co., Ltd.||Electric power supplying system for a continuous transit system by magnet (CTM)|
|US5470034 *||Mar 29, 1994||Nov 28, 1995||Westinghouse Brake & Signal Holding Ltd.||Railway track circuits|
|DE2224056A1 *||May 17, 1972||Nov 30, 1972||Signaux Entr Electriques||Title not available|
|U.S. Classification||246/34.0CT, 246/130, 246/122.00R|
|International Classification||B61L1/00, B61L1/18|