Railway Signal System

Abstract

A railway crossing signaling system in which an audio frequency signal coupled into the tracks is affected by the presence of a train near the crossing location and utilized to control operation of a conventional crossing signal relay. A relatively high impedance amplifier is transformer coupled to a section of the tracks, the latter acting as a part of a low impedance feedback path to cause oscillations at a fixed frequency determined by a high Q resonant reed filter. In preferred embodiments of the invention, oscillation is sustained via either direct connection of the feedback loop or connection through the tracks and interrupted by the shunting effect of the train, however, one embodiment of the invention relies on the presence of a train to complete the feedback circuit to produce oscillation. In all embodiments of the invention a relatively low frequency high power signal is impressed upon the track circuit through the common amplifier both to enhance oscillation and to cause frittering or establishment of the track circuit in spite of surface contamination and the like.

Description

This invention relates to signaling systems and more particularly to apparatus for controlling indicators signaling the approach of trains at railway crossings and the like.

One of the problems with systems of this type is that of achieving consistent operation under great variations of encountered conditions, for example where the condition of the track is influenced by environmental factors or where variable characteristics are exhibited by different trains which are to be monitored. Railway signal systems must be reliable and the possibility of component failure imposes frequent checking procedures, a burden and high expense for railway maintenance crews. Preferably systems of this type should be of the fail-safe variety wherein any fault in the system is recognized and signaled, however such advantage cannot be attained at the expense of introducing a large number of components in the system which may affect reliability.

Railway signaling systems which rely on an impressed signal basis are in widespread use today. Such systems characteristically operate by transmitting an oscillatory signal into the track system and detecting the presence of such signal at a remote location or the variations in same when modified by the shunting effect of a train in the vicinity. One of the faults of such systems is that extraneous noise can be introduced therein, for example by electric trains which have associated therewith a strong magnetic field that can be coupled into the track system or the connecting wires leading to the indication system.

Also in order to obtain an appreciable length of island in which the system is sensitive to the presence of a train it is necessary to have a powerful transmitter and sensitive receiver arrangement and in many instances it is necessary to isolate such islands by the insertion of insulative members in the track sections to interrupt the electrical continuity thereof. Typically such systems consume relatively large amounts of power and are subject to deterioration of components due to the heating effects therein and may be so sensitive as to be affected easily by extraneously introduced signals, for example, from power lines and the like.

Still further such prior art systems are typically frequency sensitive in that in order to obtain any measure of discrimination, frequency matching must be effected between the transmitter and receiver units. Due to the temperature sensitivity of components or even the aging characteristics thereof, frequency variations can be encountered and can only be effectively cancelled by the inclusion of costly stabilization equipment.

The apparatus of the instant invention provides improved performance in these areas in utilizing a minimum of conventional components in a specific self-sustaining oscillatory mode, which mode of operation is highly insensitive to temperature variations, extraneous signals and the effects of component aging. Essentially the signaling system comprises an amplifier circuit connected in an oscillatory mode, utilizing a section of the tracks as a portion of the feedback connection or in shunt with the feedback connection, to promote oscillation at a single specified frequency.

The system comprises a transistor amplifier coupled to the tracks by way of a transformer which matches the intermediate impedance of the amplifier with the extremely low impedance of the track circuit. A second transformer is coupled to the tracks in a similar manner, this transformer also matching impedance levels and having an output fed to a highly selective audio filter for return to the input of the amplifier in a proper phase to produce oscillations therein. The audio filter is determinative of the frequency of operation of the system and preferably is of the mechanical type comprising a contactless resonant reed filter. The track connections are arranged such that the presence of a train in the vicinity of the signaling system produces a shunting effect upon the output signal from the amplifier circuit, thereby causing loss of signal or loading and a frequency variation therein and rejection by the audio filter to prevent further oscillation.

Superimposed upon the oscillatory signal of the system is a second signal of relatively low frequency and synchronized with the main oscillatory signal to produce relatively large voltage excursions in the track circuit to assist in completion of the electrical circuit through the shunting medium. This phenomenon is known as frittering which produces lower load impedances due to the fact that the physical contact which is attained over a relatively small surface area is subject to a relatively high density of current transfer. The contact peaks are partially eroded producing a larger surface area and promoting also a better penetration of surface contaminants.

Sensing the condition of oscillation of the main circuit is a relay driver circuit of fairly conventional configuration. This circuit amplifies the oscillatory signal to a power level suitable for rectification and filtering to drive a typical crossing relay which may in turn control crossing lights or crossing gate circuits. The relay driver circuit is arranged so that a suitable oscillatory signal is required to maintain energization of the crossing relay and the remainder of the circuitry is fully AC coupled throughout to provide a fail-safe configuration of operation. Shunting of the signal in the feedback circuit primarily reduces the amplitude of the feedback signal but also may alter the frequency of operation of the system to a frequency outside the acceptance range of the audio filter as well as affect the relative phase of signals therein so that the system drops out of oscillation, which condition is detected by the relay driver circuit, causing deenergization of the crossing relay. A similar result occurs upon failure of any one component in the entire system so that any condition outside of that critical condition of stability of the system presents a fault indication.

Controls are provided for the power level supplied to the track circuit and for the gain or sensitivity of the relay driver circuit, such controls being interdependent to effect a variation in the length of island protected by the system and the sensitivity and stability of the complete system. Several variations of track interconnection are depicted whereby continuity of the track between connection locations can be verified as well as achieving various effects upon the length of island. Another variation of the system, not in the fail-safe mode but providing a reverse mode of operation is an interconnection with the track system which relies upon the presence of a train within the island of sensitivity to complete the feedback loop in order to produce oscillations for operation of the crossing relay.

It is therefore one object of this invention to provide an improved railway signaling system which is fail-safe in operation and is more reliable than previously known systems.

It is another object of this invention to provide an improved railway signaling system which operates in a single mode of oscillation and which is capable of rejecting undesired oscillatory modes.

It is a still further object of this invention to provide an improved railway signaling system in which the track circuit forms a part of the feedback loop for an oscillator, which oscillator operates at a substantially higher impedance level.

It is yet another object of this invention to provide an improved railway signaling system which monitors the rails as a part of the system and requires continuity for proper operation.

It is a yet further object of this invention to provide an improved railway signaling system which operates at a nominal frequency and includes a superimposed oscillatory signal to assure conductivity through the track circuit.

It is a yet further object of this invention to provide an improved railway signaling system in which the island of sensitivity of the system can be adjusted by various track interconnection schemes and control over the power output level and sensitivity of the system.

Other objects and advantages of the present invention will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described, the following description and the annexed drawings setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.

In said annexed drawings:

FIG. 1 is a schematic drawing in block diagram form showing the general interconnection of components in a first embodiment of track interconnection;

FIG. 2 is a schematic drawing in block diagram form of a second embodiment of track interconnection;

FIG. 3 is a schematic drawing in block diagram form of a third embodiment of track interconnection in which reliance is made upon the shunting medium for completion of the feedback loop; and

FIG. 4 is a circuit diagram of a preferred form of the invention including a fourth embodiment of track interconnection.

Referring now to FIG. 1 there is shown a section of railroad track 10 with which the apparatus of the invention is interconnected for protection of a railway crossing indicated generally by the arrow 11. The railroad track is conventionally designated as comprising a positive rail 12 and negative rail 13, such polarity designations being employed primarily for DC track signaling circuits, but not required for purposes of this invention other than to indicate compatibility or conformity to pre-existing systems. A pair of indicator devices 14 are shown at the crossing 11 which devices may comprise the usual flashing signal lights or the well known crossing gates as well as other indicators, all typically being energized by actuation of a relay 15. Interconnection with such crossing relay 15 is designated by the dashed line 16 with further interconnection between the indicator devices which normally act in unison being shown by the dashed line 18. Such interconnection lines are typically buried underground and the indicator devices 14 may be remote from the remainder of the signaling system depending upon the available space, the availability of power for the system, the environmental conditions and the like.

The main components of the system comprise a first amplifier 19 having an output connected by way of lines 20 to a transmitter or output coupling device 21 which in turn provides insertion of the system signal into the track circuit. The coupling device 21 preferably is a transformer having a relatively high impedance primary winding receiving energization from the amplifier 19 and a low impedance secondary winding providing a pair of output leads 22 connected to the positive and negative rails 12, 13 at a first location 24 therealong. The connection location 24 is determined primarily as a function of the desired operating characteristics of the system and usually falls outside the crossing 11 which the system is protecting. There is some variation possible in distance of spacing not only from the crossing 11 but also from the receiver location 25 dependent upon the sensitivity and output power of the system and the "island" or length of tracks in which reliable responses are desired.

An input coupling device 26 or transformer is used for receipt of the signals, being connected at the second location 25 and having a pair of connecting leads 28 from the positive and negative rails 12, 13 respectively, coupled to the low impedance primary winding in proper phase to sustain oscillations within the system. The coupling devices 21, 26 may further include components forming equalizers, lightening arresters or protection devices and may be either in shunt or series connection. These devices are conventional in railway signaling systems and do not significantly affect the signals therein or mode of operation of the system.

The output of the receiver coupling device 26, being the high impedance secondary winding of a transformer is connected by lines 29 to a filter 30 characterized by a high Q, highly selective response, suitable to pass substantially a single desired frequency for determining the frequency of operation of the system. The output of the filter 30 is returned to the input of the amplifier 19 thereby completing the feedback circuit which includes the tracks 10 therein by direct connection through the rails 12, 13. It will be apparent that the portions of the tracks 10 outside the connection locations 24, 25 will also have an effect upon the signal formed in the system and as previously mentioned the range of sensitivity of the system or island may be varied to some extent.

The portion of the system described including the amplifier 19, coupling devices 21, 26, rails 12, 13 and filter 30 comprise a closed loop system in which continuity is required for proper operation. Conversely, failure in any part of the system or loss of signal will cause a cessation of oscillation, the removal of the signal being positively effected under desired monitoring conditions by the shunting effect of a train in the presence of the signaling system. The transformers included in the coupling devices 21, 26 are selected to match approximately a 2 ohm impedance at the track circuit and a relatively higher impedance, on the order of fifty to several hundred ohms, in the amplifier circuit thereby providing a closed loop system with very low feedback impedance. This characteristic provides reliable and consistent results from the system, matching the shunting effect of the train which is on the order of an ohm or two or even less depending upon many conditions including the surface contamination of the tracks, the type and weight of the train thereon, the common surface area and the like. This impedance arrangement, however, provides great discrimination between the shunted and unshunted conditions produced by the train and the circuit makes it virtually impossible for noise or other signals to interfere with operation, concurrently eliminating the necessity for external filters and the like in the signal lines.

The system further includes a second amplifier 31 having an output connected to the crossing relay 15 for energization of the latter when an oscillatory signal is present in the closed loop system. This detection and indication portion of the system is connected at a convenient location in the closed loop by means of leads 32, to amplify the oscillatory signal appearing therein in order to develop sufficient power for direct energization of the crossing relay 15, the relay being deenergized in the absence of such signal indicating a fault or desired monitored condition in the system.

An independent oscillator 34 is also included as a part of the system, providing a signal output or line 35 to the first amplifier 19 at a frequency substantially lower than the normal frequency of oscillation of the system to serve two important functions. First the oscillator 34 introduces transients into the system to assure consistent operation of same and to assist in initial start-up. Since the feedback is variably affected by the presence of a shunting medium in the region of the monitored island, the transition range from the oscillatory to the non-oscillatory mode may be extremely broad and subject to temperature influences and the like. The signal introduced from the oscillator 34 alleviates this condition in providing for more consistent oscillation of the system when in the transient range and not substantially affecting the operation of same when sufficient shunting is encountered. Secondly, the signal from the oscillator 34 combines with the signal normally occurring in the closed loop system to produce a high power pulse at intervals therein determined by the frequency of the oscillator 34, which pulses cause the frittering effect upon the components forming the shunt connection, i.e., the track 10 and train electrical connection, to assure and maintain electrical continuity. Effectively the frittering increases the contact surface area in spite of surface contamination and the like reducing the effective impedance of the shunting medium, thereby providing more precise and reliable monitoring conditions.

Referring now to FIG. 4 there is shown a schematic circuit diagram of a preferred embodiment of the invention in a specific interconnection with a section of railroad tracks 36. Input power for the system is received at positive power terminals 38 and negative power terminals 39, being directly connectible to a source of power (not shown) which may be a conventional lead-acid storage battery closely associated with the equipment or alternatively a semiconductor power supply energized from available power lines. The power requirements for the system are minimal due to the use of semiconductor components and the simplified circuit arrangement as to further enhance the reliability of the system and effectively reduce maintenance procedures. The major portion of power consumed in the system is that required for substantially continuous energization of the crossing relay, which requirement is imposed by the fail-safe standard of operation.

First and second output terminals 40, 41 and first and second input terminals 42, 44 are shown for the system, these terminals being commonly referred to as transmitter and receiver terminals respectively, in the commonly employed terminology of the trade, however in the instant system such terminals are merely connection points for wires leading to the tracks 36 for establishment of the electrical island of sensitivity. In this system of interconnection the first output terminal 40 and first input terminal 42 are connected by separate lines 45, 46 to a common location 48 on the positive rail while the second output terminal 41 and second input terminal 44 are similarly connected by separate lines 49, 50 to a second location 51 on the negative rail of the track 36. It will be clear that by this connection arrangement, the output terminals 40, 41 are connected directly to the input terminals 42, 44 and in parallel connection with the section of tracks 36 such that a train in proximity to the connection locations 48, 51 will cause a shunting and thus a diminution of the electrical signal appearing in the track circuit.

The amplifier section of the system comprises three transistors 52, 54, 55 in common emitter configuration providing power gain for the input signal received on line 56. Transistor 52 receives the input signal at the base electrode thereof and is AC coupled, by way of capacitor 58, to the base electrode of transistor 54, both transistors 52, 54 being suitably biased to provide sufficient gain for the input signal. A first potentiometer 59 in the collector path of transistor 54 provides adjustment over the amplitude of signal appearing there and is connected in turn to the relay driver portion 60 of the system. A second potentiometer 61 in the collector path of transistor 54 is AC coupled by way of capacitor 62 to the base circuit of transistor 55, this potentiometer 61 being designated the track power control potentiometer and primarily determinative of the output level of the signal applied to the track circuit.

The collector electrode of transistor 55 is coupled to the primary winding of the output or transmitter transformer 64 for isolation of the system from the tracks 36 and for impedance matching purposes. The secondary winding of transformer 64 is connected to the output terminals 40, 41 of the system, one lead including a series capacitor 65 for DC isolation purposes in the event that DC signaling or other circuits are employed on the same tracks 36.

In a similar manner the signal from the track circuit is received at the input terminals 42, 44 and applied to the low impedance primary winding or the input or receiver transformer 66 by way of a series capacitor 68, again for isolation purposes. The relatively high impedance secondary winding of the input transformer 66 supplies the signal input to the filter 70 which is determinative of the frequency of operation of the system. Coupling is made by way of a resistor and capacitor network 71 and a pair of back-to-back diodes 72 in shunt connection across the input of the filter 70 to prevent overloading of same. The output of the filter 70 is connected to the base circuit of transistor 52 by lead 56, the transistor being biased or referenced to a slightly positive voltage by means of the divider consisting of resistors 74, 75, noise suppression being provided by the series connected capacitors 76.

While the filter 70 may be of any type which provides a highly selective characteristic, to produce transmission of substantially only a single frequency, preferably the filter 70 is of the mechanical type utilizing a vibrating reed 77 for providing coupling between the input and output windings thereof, such reeds 77 being resonant only at the desired frequency of operation and providing an extremely high Q factor. A filter especially suited for this purpose is the RF-20 filter manufactured by The Bramco Controls Division of Ledex, Inc., providing an operating mode at 1,200 Hz. Different frequencies of operation for the system are made possible by substitution of different frequency filters of this type and it has been determined that the most useful range of operation for the system is from 500 to 2,700 Hz.

The output of the sensitivity control potentiometer 59 is applied by way of a series resistor 78 and capacitor 79 to the base circuit of a first transistor 80 in the relay driver portion 60 of the system wherein both voltage amplification and power increase occurs. Transistor 80 and power transistor 81 are connected in common emitter configuration, with the emitter circuit of the former coupled to the base circuit of the latter by the low impedance resistor 82. An isolation transformer 84 is connected in the collector circuit of transistor 81 and provides an output voltage at its secondary winding when the system is in an oscillatory mode, which voltage is rectified in the bridge rectifier 85 and filtered by the shunt capacitor 86 in parallel with terminals 88, to which the crossing relay 89 is connected for energization of the latter. The transformer 84, rectifier 85 and filter 86 arrangement for energization of the crossing relay 89 is a standard circuit configuration required to assure fail-safe performance. In the event of fault anywhere within the system and loss of the oscillatory signal therein, no voltage will be developed across the filter capacitor 86 for energization of the relay 89 and the latter will automatically revert to a condition indicative of same. The relatively long time constant of the filter circuit provides a time lag for response so that momentary interruptions due to switching spikes or voltage transients and the like may pass without indication.

The crossing relay 89 in turn includes suitable contacts (not shown) for control of indicator lamps or gate movement and the like, such indicator circuits receiving power from any available source including the same DC voltage source which powers the signaling system. A pair of lock-out contacts 90 are also provided for the system, these being a pair of normally open contacts in series connection with the negative source voltage and with a diode 91, the latter connected to the base electrode of transistor 80 in the relay driver circuit 60. Such lock-out contacts 90 may be employed for auxiliary testing procedures and the like and the polarity of the diode 91 is arranged such that when the contacts are in a closed condition the base electrode of transistor 80 will be prevented from rising above the negative voltage level.

The signaling system further includes an independent oscillator 92 therein for superimposing a signal upon that normally oscillatory signal of the system for improving the conductivity of the track circuit and for exciting the system into operation during transient intervals. The oscillator 92 comprises a unijunction transistor 94 connected in a relaxation oscillator mode by virtue of the series resistor 95 and capacitor 96 connection at the emitter terminal thereof, such circuit developing positive voltage signals across the base resistor 98 and thus across a shunt connected capacitor 99, the latter providing some signal shaping. The capacitor 99 in turn is connected to the base circuit of the power output transistor 55 by way of a diode 100 biased to pass such positive signals, the diode otherwise providing isolation by virtue of its high back resistance.

Frequency of operation of the independent oscillator 92 is selected to be some fraction of the nominal frequency of operation of the system although not necessarily a submultiple thereof. In this embodiment of the invention such frequency is 18 Hz and such signals are caused to coincide or be in synchronization with the higher frequency signal of the system by virtue of the diode 100 connection, thereby providing a high level, high power output signal to the output transformer 64 and thus to the tracks 36 at the frequency of the independent oscillator 92. Such combined signal output at the tracks 36 is preferably on the order of one volt at over one ampere of current and is suitable to assure the track circuit connection through the shunt medium by the process of frittering previously described and in addition pulses the system by way of the input transformer 66 with a transient high voltage at the frequency of oscillation of the independent oscillator 92. As noted such high level signal is prevented from overloading the filter 70 by virtue of the diodes 72 at the input connection thereof but does produce a momentary high level signal throughout the system.

By virtue of the track connections shown in FIGS. 1 and 4 a shunting of the feedback signal is effected by the presence of a train in the location of the electrical island of sensitivity and in fact for some distance on either side of the island. This extension of sensitive area from those locations of direct connection 24, 25, 48, 51 to the railroad tracks 10, 36 is a function of the power supplied to the system, the sensitivity of the relay driver portion 60 of the circuit and the particular track connection utilized.

An appreciation of this distance of monitoring can be obtained in part from an understanding of the operational adjustment procedures developed for this system. Thus, for example, once the system has been physically interconnected with the tracks 10, a 0.06 ohm shunt 101 may be placed across the tracks at location 102, outside of the location 25 of physical connection, and the following adjustments can be made. With the sensitivity potentiometer 59 set at midpoint on its scale the track power control potentiometer 61 may be adjusted upwardly from the minimum level to a point where the crossing relay 89 energizes to set a level at which detection is desired. The shunt 101 should then be moved from location 102 to location 25, approximately at the location of the physical connection to the tracks 10, at which point the relay should de-energize and provide the desired indication. The sensitivity control 59 may now be rotated throughout its range noting the dropout and pull in points of the relay 89 with the island length being the longest at the point where the relay energizes. Conversely, the opposite setting of the sensitivity control 59 will shorten the island distance to the minimum level.

In either configuration of the system shown in FIGS. 1 and 4 the presence of a train in the island of sensitivity acts to shunt out the signal in the feedback path to disrupt the oscillatory mode of system. It is also significant to note, in the FIG. 1 embodiment of the invention, that proper operation of the system depends upon electrical conduction through the rails 12, 13 between the connection points 24, 25, thereby operating as a check upon the continuity of the rail system. In the FIG. 4 embodiment the output terminals 40, 41 and input terminals 42, 44 are directly connected, with the tracks 36 in shunt connection thereacross, so that no continuity in the track circuit is required.

FIG. 2, however, depicts another embodiment of this system in which the rails 104, 105 are monitored and in which the presence of a train shunts out the signal in the feedback circuit, such system falling into the category of a "wrap-around" circuit in which continuity in the system is required for proper operation. In this embodiment of the invention one lead 106 of the output transformer 108 and one lead 109 of the input transformer 110 are connected to individual rails 104, 105, but at a common location 111 along the tracks by way of the conventional arrester and equalizer devices 112 previously mentioned. The remaining leads 114, 115 of the output 108 and input 110 transformers respectively are also coupled to individual rails 104, 105 at the second location 116 along the tracks and outside the crossing 118. It will be noted that the output and input transformers are coupled by way of the electrical connections through the rails 104, 105 and therefore require continuity of the rail system for proper operation. Similarly in this type of system the presence of a train in the location of the crossing 118 produces a shunting effect upon the signal. These individual non-paired connections to the rail are suitable for either the short or long islands and are characterized as providing a short ring by hookup.

FIG. 3 depicts yet another embodiment of the invention significantly different from those previously described in that it is required that the train be present in the island of sensitivity in order to complete the feedback circuit and produce an oscillatory signal for detection by the relay driver and for energization of the crossing relay 120. In this embodiment of the invention one terminal of the output transformer 121 and one terminal of the input transformer 122 are directly connected by way of a short length of jumper wire 124 while the remaining terminals of the transformers are connected by leads 125, 126 to the positive and negative rails 128, 129 at spaced locations 130, 131 therealong. This system provides a reversed mode of operation from those previously described but does require conductivity through the track circuit to complete the feedback loop, relying also on the presence of the shunt medium for this purpose. Once again, arrester devices 132, 134 are employed, having no effect upon system operation and modifications can be made to the track circuit by the insertion of insulated joints at locations 135, 136 in either rail to further isolate the island of sensitivity. Oscillation of the system is similar to the other embodiments of the invention described and it will be appreciated that the shock effect of the independent oscillator may be advantageous to start oscillation of the system and assure conductivity upon the approach of a train, thereby reducing transition effects.

In all of the embodiments of the system described it is possible to change the nominal frequency of operation merely by the substitution of a different frequency filter 70 therein, in most cases not even necessitating trimming of the sensitivity 59 or track power control 61 potentiometers. Such arrangement is advantageous when several systems are employed along a common track and so closely adjacent as to overlap in their sensitive ranges. The critical pass band of the filter 70 prevents receipt of interfering signals from adjacent stations and similarly may be chosen to reject interfering noise signals developed from passing trains, nearby power lines and the like. Utilization of insulated joints can be made if more definite discrimination between adjacent stations is required but preferably, so as not to affect DC signaling circuits and the like, the frequency determinative method is preferred.

Claims

I, therefore, particularly point out and distinctly claim as my invention:

1. Apparatus for establishing an electrical island along a railroad track sensitive to the presence of a shunting medium therein and for signaling such shunting condition, comprising input and output transformers, said transformers being coupled to the track at first and second locations to establish the electrical island, an amplifier for energizing said output transformer, said amplifier having said input transformer connected at the input thereof forming an oscillator system in which oscillations are affected by a shunting medium in the electrical island, a resonant reed filter in connection at the input of said amplifier for restricting oscillation to a predetermined frequency, an oscillator coupled to said amplifier, said oscillator having a lower frequency oscillation than said predetermined frequency and producing a signal at said lower frequency, said signal being supplied to said amplifier for amplification, and means for monitoring the oscillatory state of said amplifier and for providing indications thereof.

2. Apparatus as set forth in claim 1 wherein said monitoring means comprises a second amplifier coupled to said first amplifier, a transformer energized by said second amplifier, a bridge rectifier connected to the output of said transformer for converting the AC signal to pulsating DC, a capacitor in connection at the output of said bridge rectifier for smoothing the DC voltage, and a crossing relay in shunt connection with said capacitor, said relay being operative to control railway crossing signal devices.

3. Apparatus as set forth in claim 2 wherein said first amplifier comprises first and second stages of transistor amplifiers connected in common emitter configuration, and a third power amplifier stage having said output transformer in the collector path thereof, and further including a track power control potentiometer in connection between said second and third stages for variation of the signal applied to the tracks, said amplifier stages being AC coupled to provide a fail-safe operation.

4. Apparatus as set forth in claim 3 wherein said second oscillator circuit comprises a unijunction transistor connected in oscillatory configuration for charging a capacitor in the base circuit thereof, and a diode connecting said capacitor and the base electrode signal circuit of said third amplifier stage for injection of the oscillatory signal, said diode providing synchronization of the signals of said oscillator circuit and said first amplifier circuit.

5. Apparatus as set forth in claim 4 wherein the output circuit and input circuit of said input and output transformers respectively include a high value capacitor connected to the tracks for isolation from DC signals.

6. Apparatus as set forth in claim 5 further including a potentiometer in the collector path of said second stage amplifier, the slider of said potentiometer being connected to the input of said relay driver circuit for varying the sensitivity of the system.