US3614418A - Railroad grade crossing protection system - Google Patents

Railroad grade crossing protection system Download PDF

Info

Publication number
US3614418A
US3614418A US3614418DA US3614418A US 3614418 A US3614418 A US 3614418A US 3614418D A US3614418D A US 3614418DA US 3614418 A US3614418 A US 3614418A
Authority
US
Grant status
Grant
Patent type
Prior art keywords
voltage
means
output
train
system
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
Inventor
Richard V Pell
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
MARQUARDT IND PRODUCTS CO
Original Assignee
MARQUARDT IND PRODUCTS CO
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Grant date

Links

Images

Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B61RAILWAYS
    • B61LGUIDING RAILWAY TRAFFIC; ENSURING THE SAFETY OF RAILWAY TRAFFIC
    • B61L29/00Safety means for rail/road crossing traffic
    • B61L29/24Means for warning road traffic that a gate is closed or closing, or that rail traffic is approaching, e.g. for visible or audible warning
    • B61L29/28Means for warning road traffic that a gate is closed or closing, or that rail traffic is approaching, e.g. for visible or audible warning electrically operated
    • B61L29/286Means for warning road traffic that a gate is closed or closing, or that rail traffic is approaching, e.g. for visible or audible warning electrically operated using conductor circuits controlled by the vehicle

Abstract

A railroad crossing warning indicator which predicts the time of arrival of trains to a grade crossing is described. Two voltages are derived from the track reactance magnitude and the impedance magnitude and are both indicative of the distance of the train. By summing the difference between the impedance voltage and the reactance voltage with the impedance voltage, a new distance voltage is obtained whereby errors are reduced due to the nonlinearity of the signals due to ballast resistances in the tracks.

Description

United States Patent lnventor Richard V. Pell [56] A l N 3 5 1 UNITED STATES PATENTS I pp o.

Filed Feb 26 970 3,246,143 4/1966 Steele et al. 6. 246/128 Patented Oct. 19, 1971 Primary Examiner--Arthur L. La Point Assignee Marquardt Industrial Products Co. Assistant Examiner-George H. Libman Cucarnoug a, Calif.

RAILROAD GRADE CROSSING PROTECTION References Cited Attorney-Robert E. Geauque ABSTRACT: A railroad crossing warning indicator which predicts the time of arrival of trains to a grade crossing is described. Two voltages are derived from the track reactance 2 D magnitude and the impedance magnitude and are both indicaauns rawmg tive of the distance of the train. By summing the difference U.S.Cl 246/128 between the impedance voltage and the reactance voltage Int.Cl B61l29/32 with the impedance voltage, a new distance voltage is ob- Field of Search. 246/126, tained whereby errors are reduced due to the nonlinearity of 128, 130 the signals due to ballast resistances in the tracks.

-V L OR/sA/ P i l2 TE L i rm ur l Owll i 1 21 :"iJH/E/i fiqA/ofiwss 26 52' r-MlLQfi/[A' war/m 3o AMA/r005 8 P/MSC 0zoscwmw \WPCT JuMM/A/G 56 A/EW/i 4 H/F/EA I 29m,

CNVIIPWM?) AIM/ 4 75/? ZLDZ'E/Jx) QUAD/P412485 A/VEAT/A/G 1 28- 0555670? ZJF/C'WEA/f/KITO/P cu M l/m6 :g 5g

54 a/sowM/A/qm so [JAKE 56 1 Q Q JUMMWG AMEN/75K 4MP1. //7[/? 44 46 i Pfizer/W5 KIM/ 2mm: AELAY VOLTAGL (OM/$424M? filMpL/F/ffi RAILROAD GRADE CROSSING PROTECTION SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to warning systems for railroad grade crossings and more particularly to an improvement in warning predictor systems used to predict the time of arrival of an approaching train.

2. Discussion of the Prior Art A typical grade crossing predictor is set forth in U. S. Pat. No. 3,246,143 and U.S. Pat. application Ser. No. 807,626, filed Mar. 17, 1969, by the same inventor and assigned to the same assignee as this invention. Much of the same systems electronics of that patent are used in the invention discussed herein.

The system of the above patent provides a railroad crossing warning system whereby delay to cross traffic is minimized. This is achieved in an arrangement wherein the railroad track is considered as a shorted transmission line in which the short is provided by the train. An alternating current signal which is a substantially constant current level is applied to the tracks at the location of the grade crossing. The voltage existing across the tracks as the train, and therefore the short, approaches the grade crossing, will diminish. Thus, the amplitude of this voltage provides a measure of the distance of the train from the crossing while the rate at which this voltage diminishes provides a measure of the velocity of the train. With these parameters it becomes possible to estimate the time of the trains arrival at the crossing. Knowing the time of arrival, the system can start warning signals at such a time as will provide the least possible delay to cross traffic. The signal representative of distance and the signals derived therefrom representative of velocity are combined to provide a third voltage representative of the time required for the train to arrive at the railroad grade crossing.

It has been found that the input impedance of the shorted railroad track section, having infinitely high ballast resistance, varies linearly with track length. The Grade Crossing Predictor, as set forth in the above patent, for example, uses this principle to develop a voltage which is the measure of the distance of the train to the predictor probe location. The voltage is derived from the reactance component of the input impedance. The rate at which this voltage diminishes as a train approaches, provides a measure of the speed of the train. These two voltages are then combined to estimate the time of the trains arrival at the crossing. Knowing the time of arrival of the train, the aforesaid system can initiate warning signals before the arrival thereof.

Since, in actual practice, ballast resistance is low enough to cause the input impedance, and in particular the reactance component, to vary nonlinearly with track length, an error is introduced into the distance voltage and thus the speed voltage. These two errors cause the predictor to err in the estimate of the arrival time of the train, thus as the ballast decreases, the error increases. Thus, a need has arisen to reduce the error in the estimate of arrival time of a train when ballast resistance is low.

The system as described in the copending application makes use of a second distance voltage which differs from the first in that it is developed from the impedance magnitude of the input impedance. It is this second voltage which is used to measure the speed of the train. The first distance voltage is used to measure the distance to a train.

SUMMARY OF THE INVENTION Briefly described, the distance-to-train voltage (E is developed in the computing circuit by two other distance voltages. One of the two voltages which is the reactance voltage (B is developed from the reactive component of the track input impedance. The other voltage is derived from the impedance component track input impedance magnitude. Because of the low ballast resistance, neither of these voltages provide an acceptable voltage due to the nonlinearity thereof.

DESCRIPTION OF THE DRAWINGS These and other objects, features and advantages will become more apparent to those skilled in the art when taken into consideration with the following detailed description, wherein like reference numerals indicate like and corresponding parts throughout the several views, and wherein:

FIG. I is a block diagram of the preferred embodiment of the invention; and

FIG. 2 is a graph of the voltage versus distance and the error reduction linearity realized by this invention.

DESCRIPTION OF ONE PREFERRED EMBODIMENT Turning now to FIG. 1, there is shown a block diagram of the preferred embodiment of this invention. The train I0 has a motion in a direction represented on a pair of track rails 12.

The train is at a distance L from the origin point P, P, which represents the location of a grade crossing, for example. The train motion occurs from left to right. Thevelocity V and the acceleration A factors are, therefore, represented as going from left'to right on the drawing. The method of computing the time of arrival is set forth fully in the aforesaid U. S. Pat. No. 3 ,246,l43 and in the copending application aforesaid.

A computer, in accordance with the aforesaid, includes an oscillator 16 which oscillates at a suitable frequency. The output of the oscillator 16 is applied to excite a power amplifier 22. A resistor 18 connects one side of the power amplifier to one of the rails at a point P. The other side of the power amplifier 22 connects to the other rail P. The power amplifier 22, together with the resistor 18, comprises a constant current generator. This delivers an input to the track at substantially a constant current.

It should be appreciated that as the train 10 approaches the points P, P on the track to which current from the constant current generator is applied, the impedance of the tracks looking toward the train from these points is continuously being diminished. Thus, the train comprises a short across the tracks 12, which is moved toward the points P, P. With current being maintained constant,.the voltage at the points P, P will continuously decrease to a minimum when the train reaches the points P, P. Therefore, by measuring the voltage across the tracks 12, an indication is obtained of the distance of the train I0 from the points at which the voltage is impressed. The change, with respect to time of this voltage, can provide velocity information and a second derivative of this voltage information provides information as to the acceleration of the train 10.

Accordingly, a narrow band-pass amplifier 26 centered at the frequency of the oscillator 16, which is connected to the same points of the tracks 12 as the constant current generator, receives a voltage representative of length of track L or distance between the train 10 and the points P, P. This voltage is an alternating current which is modulated by the motion of the train I0 toward the points P, P.

The output of the band-pass amplifier 26 is applied to a quadrature detector 28 which also has a reference input applied from the oscillator 16 through a phase shift network 30. The output of the quadrature detector 28 is applied to a summing amplifier 38. The output of the band-pass amplifier 26 is also coupled to an amplitude detector 32. The amplitude detector 32 provides a DC voltage proportional to the impedance of the track 12. The output of the amplitude detector 32 provides a voltage E developed by the track input impedance. The output of the quadrature detector 28 provides a voltage E which is developed from the reactive component of the track .input impedance. The -E,, voltage and the 1E voltage are summed in the summing amplifier 33 to produce E. #ZE E Circuit 34 provides the rate of change of that voltage to produce E =dE /dt. The output of the circuit 34 is coupled through an amplifier 36 to a summing amplifier 38, where it is summed with E The summing amplifier 38 receives the time rate of change E of the linearized distance voltage E from the output of the differentiating circuit 34 which is equal to the speed of the train 10. The output of the summing amplifier 38 is connected to a high gain amplifier 40. The output of the high gain amplifier is applied to an amplitude comparator 42 wherein it is compared with a signal from the reference voltage source 44. The output of the amplitude comparator 42 is connected to a relay amplifier 46 which operates the warning relay when the signal applied into it has a sufficient magnitude.

To complete an operative embodiment of the system, an override circuit is also provided, and this includes an amplitude discriminator 52 which receives the output from the quadrature detector 28 and compares it to the output of a reference voltage source 54. The output of the amplitude discriminator 52 is connected to a relay amplifier 56 which drives a minimum distance override relay 58. The input to the differentiator circuit 34 and summing amplifier 38 are voltages proportional to the distance L between the train and the excitation points P and P. When differentiated, this voltage gives a voltage proportional to train speed. The output of the quadrature detector 28 is a voltage proportional to the reactance component across the track which is a measure of the distance to a train from points I and P.

FIG. 2 illustrates the difference between the distance voltage derived from the reactance magnitude provided by the quadrature detector 28 and the distance voltage derived from the impedance magnitude provided by amplitude detector 32. The sum of distance voltage E derived from the reactance magnitude and the time rate of change of the linearized distance voltage E derived from the reactance and impedance magnitudes is provided by the summing amplifier 38.

When the ballast resistance is very high (R the two distance voltages have the same slope. When the ballast resistance is decreased (R -1.5 ohms, for example, lumped at the predictor) it can be readily seen that the slope of the impedance magnitude is much improved over the slope of the reactance magnitude, as shown in the two graphs in FIG. 2. Thus, the error in estimate of the arrival of a train by the predictor is also much improved. The reason that the impedance magnitude provided by amplitude detector 32 is less affected by low ballast resistance than the reactance magnitude is apparent in the following example:

Z =R +J X If we assume that R =0.5X, which provides a high ballast resistance condition, then Z ,,=l.l2 X 63.4 If we assume a low ballast resistance condition (R, 2 X) where the ballast resistance is in parallel with Z then therefore, the impedance magnitude Z ,,=0.83/ l .12 or a 26 percent reduction in its magnitude while the reactance magnitude 4 .83 sin 41.6 1.12 sin 634 IF DT' DX by subtracting, in effect, the E distance voltage from the E voltage and adds the difference back to the E a more ideal and linear slope is generated as shown in FIG. 2. This slope is v nearly as linear at times as the R, slope. This improved slope gives a more correct calculation of train speed and thus warning time at grade crossing is more nearly correct.

Thus, there has been provided by the improvements set forth herein a time of arrival predictor computer which has a lower error as compared to the prior art systems. The output, as provided by this predictor from summing amplifier 38, is sent through a high gain amplifier 40 and compared in a comparator 42 to a reference voltage provided by 44. If the sum of these voltages is above the reference voltage, then the relay amplifier 46 enables a relay 48, which, in turn, either sounds an alarm or lowers a crossing gate, or the like.

Having thus described but one preferred embodiment of this invention, what is claimed is:

l. in a system for deriving from railroad tracks information for predicting the time required for arrival at a given location of a distant train which is moving on said track towards said location, comprising:

means for applying an AC signal of a constant current level on said track from said location;

means for deriving a first voltage proportional to the reactance component across said tracks indicative of the distance of said train from said location;

means for providing a second voltage proportional to the impedance component across said tracks;

means for providing a voltage developed by the difference between said first voltage and said second voltage;

means for combining said difierence voltage with said first voltage to provide a linearized distance voltage;

means for differentiating said linearized distance voltage to obtain a voltage representative of the instantaneous speed of said train; and

means for combining said first voltage and said differentiated voltage to provide a third voltage which is a function of the time required for said train to arrive at said given location.

2. The system as defined in claim I and further comprising means for utilizing said third voltage for operating a warning device at said location.

3. The system as defined in claim 1 wherein said means for deriving said first voltage is a quadrature detector.

4. The system as defined in claim 1 wherein said means for deriving said second voltage is an amplitude detector.

5. The system as defined in claim 1 wherein said means for differentiating is an operational amplifier differentiator.

6. The system as defined in claim 1 and further comprising:

means for providing a reference voltage; and

comparator means responsive to said reference voltage and said third voltage provided by said combining means, said comparator means being adapted to provide an output when said third voltage exceeds said reference voltage.

7. The system as defined in claim 6 and further comprising means for utilizing the output voltage of said comparator means for operating a warning device at said location.

8. In a system for predicting the time of arrival of a train on a track comprising:

a source of AC signals at a constant current level, said source being coupled across said tracks at a selected location;

a quadrature detector adapted to receive signals from said track at said selected location, said quadrature detector being adapted to provide an output voltage proportional to the reactance component across said track;

an amplitude detector adapted to receive signals from said track at said selected location, said amplitude detector being adapted to provide an output voltage proportional to the impedance component across said tracks;

a first summing amplifier responsive to said amplitude detector and said quadrature detector for providing a voltage difference between the impedance output voltage and the reactance output voltage and adding this difference to the impedance output voltage;

a differentiator circuit responsive to the output voltage of said first summing amplifier adapted to provide an output indicative of the instantaneous speed of said train; and

a second summing amplifier responsive to the output voltage of said differentiator circuit and the output voltage of said quadrature detector, said summing amplifier being adapted to provide an output voltage which is a function of the time required for said train to arrive at said location.

9. The system as defined in claim 6 and further comprising means for utilizing the output voltage of said summing amplifier for operating a warning device at said location.

10. The system as defined in claim 6 and further comprising:

a reference voltage source adapted to provide an output voltage of a predetermined level; and

an amplitude comparator being responsive to the output voltage of the said summing amplifier and to the output voltage of said reference voltage source and being adapted to provide an output signal when the voltage provided by said summing amplifier exceeds the voltage provided by said reference voltage source.

11. The system as defined in claim 8 wherein said source of AC signals includes:

an oscillator adapted to provide an output signal at a predetermined frequency; and

a power amplifier coupled between said oscillator and said tracks.

12. The system as defined in claim 11 and further including a band-pass amplifier coupled between said track and said quadrature detector and said amplitude detector.

13. The system as defined in claim 12 and further comprising means for utilizing the output voltage of said amplitude comparator for operating a warning device on said location.

14. A system for determining the distance to a vehicle on a railroad track which has an electrical current thereon, including;

means coupled to said track for generating a first voltage indicative of the track input impedance;

means coupled to said track for generating a second voltage indicative of the reactance component of said track input impedance;

means for generating a third voltage proportional to the difference between said first voltage and said second voltage; and

means for combining said third voltage with said first voltage to provide a fourth voltage indicative of said distance.

15. The system as defined in claim 14 wherein said means for generating said first voltage comprises an amplitude detec- 1101'.

16. The system as defined in claim 14 wherein said means for generating said-second voltage comprises a quadrature detector.

17. The system as defined in claim 14 wherein:

said means for generating said first voltage comprises an amplitude detector; and

said means for generating said second voltage comprises a quadrature detector.

Claims (17)

1. In a system for deriving from railroad tracks information for predicting the time required for arrival at a given location of a distant train which is moving on said track towards said location, comprising: means for applying an AC signal of a constant current level on said track from said location; means for deriving a first voltage proportional to the reactance component across said tracks indicative of the distance of said train from said location; means for providing a second voltage proportional to the impedance component across said tracks; means for providing a voltage developed by the difference between said first voltage and said second voltage; means for combining said difference voltage with said first voltage to provide a linearized distance voltage; means for differentiating said linearized distance voltage to obtain a voltage representative of the instantaneous speed of said train; and means for combining said first voltage and said differentiated voltage to provide a third voltage which is a function of the time required for said train to arrive at said given location.
2. The system as defined in claim 1 and further comprising means for utilizing said third voltage for operating a warning device at said location.
3. The system as defined in claim 1 wherein said means for deriving said first voltage is a quadrature detector.
4. The system as defined in claim 1 wherein said means for deriving said second voltage is an amplitude detector.
5. The system as defined in claim 1 wherein said means for differentiating is an operational amplifier differentiator.
6. The system as defined in claim 1 and further comprising: means for providing a reference voltage; and comparator means responsive to said reference voltage and said thiRd voltage provided by said combining means, said comparator means being adapted to provide an output when said third voltage exceeds said reference voltage.
7. The system as defined in claim 6 and further comprising means for utilizing the output voltage of said comparator means for operating a warning device at said location.
8. In a system for predicting the time of arrival of a train on a track comprising: a source of AC signals at a constant current level, said source being coupled across said tracks at a selected location; a quadrature detector adapted to receive signals from said track at said selected location, said quadrature detector being adapted to provide an output voltage proportional to the reactance component across said track; an amplitude detector adapted to receive signals from said track at said selected location, said amplitude detector being adapted to provide an output voltage proportional to the impedance component across said tracks; a first summing amplifier responsive to said amplitude detector and said quadrature detector for providing a voltage difference between the impedance output voltage and the reactance output voltage and adding this difference to the impedance output voltage; a differentiator circuit responsive to the output voltage of said first summing amplifier adapted to provide an output indicative of the instantaneous speed of said train; and a second summing amplifier responsive to the output voltage of said differentiator circuit and the output voltage of said quadrature detector, said summing amplifier being adapted to provide an output voltage which is a function of the time required for said train to arrive at said location.
9. The system as defined in claim 6 and further comprising means for utilizing the output voltage of said summing amplifier for operating a warning device at said location.
10. The system as defined in claim 6 and further comprising: a reference voltage source adapted to provide an output voltage of a predetermined level; and an amplitude comparator being responsive to the output voltage of the said summing amplifier and to the output voltage of said reference voltage source and being adapted to provide an output signal when the voltage provided by said summing amplifier exceeds the voltage provided by said reference voltage source.
11. The system as defined in claim 8 wherein said source of AC signals includes: an oscillator adapted to provide an output signal at a predetermined frequency; and a power amplifier coupled between said oscillator and said tracks.
12. The system as defined in claim 11 and further including a band-pass amplifier coupled between said track and said quadrature detector and said amplitude detector.
13. The system as defined in claim 12 and further comprising means for utilizing the output voltage of said amplitude comparator for operating a warning device on said location.
14. A system for determining the distance to a vehicle on a railroad track which has an electrical current thereon, including; means coupled to said track for generating a first voltage indicative of the track input impedance; means coupled to said track for generating a second voltage indicative of the reactance component of said track input impedance; means for generating a third voltage proportional to the difference between said first voltage and said second voltage; and means for combining said third voltage with said first voltage to provide a fourth voltage indicative of said distance.
15. The system as defined in claim 14 wherein said means for generating said first voltage comprises an amplitude detector.
16. The system as defined in claim 14 wherein said means for generating said second voltage comprises a quadrature detector.
17. The system as defined in claim 14 wherein: said means for generating said first voltage comprises an amplitude detector; and said means for generatIng said second voltage comprises a quadrature detector.
US3614418A 1970-02-26 1970-02-26 Railroad grade crossing protection system Expired - Lifetime US3614418A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
US1437370 true 1970-02-26 1970-02-26

Publications (1)

Publication Number Publication Date
US3614418A true US3614418A (en) 1971-10-19

Family

ID=21765089

Family Applications (1)

Application Number Title Priority Date Filing Date
US3614418A Expired - Lifetime US3614418A (en) 1970-02-26 1970-02-26 Railroad grade crossing protection system

Country Status (1)

Country Link
US (1) US3614418A (en)

Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3977634A (en) * 1975-06-09 1976-08-31 Safetran Systems Corporation Computer for motion sensing device setup
US4306694A (en) * 1980-06-24 1981-12-22 American Standard Inc. Dual signal frequency motion monitor and broken rail detector
US4365777A (en) * 1979-08-17 1982-12-28 Modern Industries Signal Equipment, Inc. Train approach detector
US20070074581A1 (en) * 2005-10-03 2007-04-05 General Electric Company Method and system for calculating railroad track ballast resistance

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3246143A (en) * 1963-09-30 1966-04-12 Southern Pacific Company Railroad grade crossing protection system

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3246143A (en) * 1963-09-30 1966-04-12 Southern Pacific Company Railroad grade crossing protection system

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3977634A (en) * 1975-06-09 1976-08-31 Safetran Systems Corporation Computer for motion sensing device setup
US4365777A (en) * 1979-08-17 1982-12-28 Modern Industries Signal Equipment, Inc. Train approach detector
US4306694A (en) * 1980-06-24 1981-12-22 American Standard Inc. Dual signal frequency motion monitor and broken rail detector
US20070074581A1 (en) * 2005-10-03 2007-04-05 General Electric Company Method and system for calculating railroad track ballast resistance
US7295017B2 (en) 2005-10-03 2007-11-13 General Electric Company Method and system for calculating railroad track ballast resistance

Similar Documents

Publication Publication Date Title
US4022058A (en) Apparatus for determining the arrival time of alternating signals
US5072900A (en) System for the control of the progression of several railway trains in a network
US3729668A (en) Aparatus for controlling the displacement of an object between any two points
US4132991A (en) Method and apparatus utilizing time-expanded pulse sequences for distance measurement in a radar
US5054315A (en) Coding of the value of several quantities measured in a tire
US3881167A (en) Method and apparatus to maintain constant phase between reference and output signals
US4129276A (en) Technique for the detection of flat wheels on railroad cars by acoustical measuring means
US4499417A (en) Determining location of faults in power transmission lines
US5341144A (en) Vehicular cruise control system and radar system therefor
US4117529A (en) Broken rail detecting track circuits
US6951132B2 (en) Rail and train monitoring system and method
US3775742A (en) Vehicle detection system
Westeon et al. Monitoring vertical track irregularity from in-service railway vehicles
US4422322A (en) Method and system for measurement of road profile
US3610920A (en) Apparatus and method for deriving a uniform time warning
US20050076716A1 (en) Method and apparatus for detecting guideway breaks and occupation
US6405141B1 (en) Dynamic track stiffness measurement system and method
Chen et al. Identification of a driver steering model, and model uncertainty, from driving simulator data
US3395341A (en) Method and apparatus for detecting the velocity of moving metallic masses by means of phasedisplacements produced in magnetic windings
US3810099A (en) Means for providing a vehicle control signal containing direction and speed information
US5381700A (en) Train analysis system enhancement having threshold adjustment means for unidentified wheels
US3343167A (en) Object detection system
US3581084A (en) Piezoelectric wheel-axle detector
US5736695A (en) Device for detecting position
US4151969A (en) System for selectively determining the location of a railway car moving along a railway track