US3609239A

US3609239A - Fail-safe pulse repeater

Info

Publication number: US3609239A
Application number: US885084A
Authority: US
Inventors: Richard D Campbell
Original assignee: Westinghouse Air Brake Co
Current assignee: Hitachi Rail STS USA Inc
Priority date: 1969-12-15
Filing date: 1969-12-15
Publication date: 1971-09-28
Anticipated expiration: 1988-09-28
Also published as: JPS5012922B1

Abstract

The invention relates to a fail-safe signal repeater for receiving and repeating a signal from and to an electrically continuous rail. The signal repeater includes an input circuit electrically coupled to the electrically continuous rail to receive the signals to be repeated. A signal-repeating output circuit is electrically coupled to the rail to provide the repeated signal. Finally there is a control circuit which is electrically coupled respectively to the input circuit and the signal-repeating output circuit to thereby initiate the repeated signal while simultaneously providing a period of nonresponse for the repeater for a predetermined period of time after the signal to be repeated is received and repeated.

Description

United States Patent Primary Examiner-Kathleen H. Claffy Assistant Examiner-William A. Helvestine [72] lnventor Richard D. Campbell Harmarville, Pa.

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AttrneysH. A. Williamson, A. G. Williamso Sotak 8 a w SDS 0 d N m L n Pwm fla AFP 11:1 25 224 [.11

[73] Assignee Westinghouse Air Brake Company Swissvale, Pa.

ABSTRACT: The invention relates to a fail-safe signal repeater for receiving and re [54] FAIL-SAFE PULSE REPEATER Claims, 1 Drawing Fig. [52] US.

peating a signal from and to an electrically continuous rail. The signal repeater includes an input circuit electrically coupled to the electrically continuous rail to receive the signals to be repeated. A signal-repeating output circuit is electrically coupled to the rail to provide the re- [51] lnt.Cl........

[] Field ofSearch.......

178/70 R, T, 70 TS, 71

, 2 peated signal. Finally there is a control circuit which is electri- [56] References Cited UNITED STATES PATENTS 3,462,552 8/1969 Dobermann.................. 3,499,985 3/1970 Rowlands vv v v v 11 ra l/7 FAIL-SAFE PULSE REPEATER This invention relates to a signal repeater for use in electrically continuous rail track circuits. More specifically this invention relates to a fail-safe signal repeater for receiving and repeating a signal from and to an electrically continuous rail. The signal repeater includes an input circuit electrically coupled to the electrically continuous rail to receive the signals to be repeated. A signal-repeating output circuit is electrically coupled to the rail to provide the repeated signal. Finally there is a control circuit which is electrically coupled respectively to the input circuit and the signal-repeating output circuit to thereby initiate the repeated signal while simultaneously providing a period of nonresponse for the repeater for a predetermined period of time after the signal to be repeated is received and repeated.

in recent years the trend in track circuits and signaling systems that employ track circuits has been toward larger track circuits employing electrically continuous rail. in many areas of the country there are miles upon miles of rail where the traffic density is low and the employment of elaborate signaling systems for these areas is prohibitively expensive, and as a consequence no signaling is provided. Furthermore, all presently known signaling systems require line wires and/or breaks in one or both of the rails to establish signaling through the rails. The presence of signal attenuation in the rails places limitations on the length of the track circuits, thereby increasing the number of interruptions that must be made in the rails.

Therefore, if there could be an inexpensive uncomplicated signaling system that could employ the electrically continuous rails for stretches of to miles or more without breaks in the rail and operate in a fail-safe manner, such a system would answer the longfelt need of the railroading industry in these areas. Past signaling systems were dogged by the ever-present problem of signal attenuation which would require an interruption in one or both of the rails in order that signals induced at one end of the track circuit be repeated many times and eventually transmitted to the other end. To the longfelt need above noted this invention provides the answer by providing a unique signal repeater that operates in a fail-safe manner and requires no breaks in the rail. The repeater of this invention now makes possible the attainment of signaling systems that may be of great length which do not require the use of line wires or require breaks in the rail.

It is therefore an object of this invention to provide a failsafe signal repeater that may be electrically coupled across a pair of electrically continuous conductors, such as track rails.

Another object of this invention is to provide a pulse signal repeater that will never fire spontaneously. Yet another object of this invention is to provide a pulse signal repeater in which the internal gain must not increase so that weaker signals might trigger the repeater.

Still another object of this invention is to provide a pulse signal repeater which has a period of nonresponse to received pulses which occurs after receiving a pulse and transmitting a repeated pulse.

Another object of this invention is to provide a pulse signal repeater in which the period of nonresponse never gets shorter or becomes zero.

Another object of this invention is to provide a pulse signal repeater which has an output pulse that never increases in height even when the output of the repeater is shorted.

in the attainment of this invention there is provided a signal repeater which receives and repeats a signal from and to a pair of electrically continuous rails. The repeater includes an input circuit which is electrically coupled to the rails to receive a signal desired to be repeated. The input circuit includes an amplifier which is transformer coupled to the rails to thereby receive and amplify the signal from the rails. The amplifier has an output electrically coupled to a control circuit. The control circuit is electrically coupled respectively to the input circuit and the signal repeating output circuit. Specifically, the control circuit includes a triggered blocking oscillator electrically coupled to the amplifier output of the input circuit to be triggered thereby. The blocking oscillator has an output signal electrically coupled to the signal-repeating output circuit. The blocking oscillator output signal has a time length equal to the predetermined period of time aforementioned. The blocking oscillator output signal provides the period of nonresponse for the signal repeater. The signal-repeating output circuit includes therein a circuit which has a stored signal therein as well as a switching circuit. The switching circuit is electrically coupled respectively to the blocking oscillator output and the circuit with the stored signal therein. The switch circuit is responsive to the appearance of the blocking oscillator output signal to allow the stored signal to be delivered to the electrically continuous rail as the repeated pulse.

Other objects and advantages of the present invention will become apparent from the ensuing description of illustrative embodiments thereof, in the course of which reference is had to the accompanying drawing in which the single FIGURE depicts a circuit diagram of the fail-safe pulse repeater of the subject invention.

A description of the above embodiment will follow and then the novel features of the invention will be presented in the appended claims.

Reference is now made to the drawing which illustrates in block diagram form the basic component of a system which could typically embody the invention to be described. in one of the most elementary systems embodying the invention a transmitter coupled across the rails ll and 12 would meet the basic need. The transmitter 23 is electrically coupled across the rails via

leads

38, 39. Across the rails are shown, but not referenced with numerals, a plurality of capacitors between the transmitter 23 and a unit 22, which is a repeater and the subject of this invention, which will repeat a signal delivered from the transmitter 23 to the repeater 22. The repeater 22 will then amplify the signal and retransmit a repeated pulse along the rails toward the right where it will be repeated by an additional repeater which is coupled across the rails and bears the designation R. At the right-hand end of the track circuit is a receiver 57 which receives the signal originally generated by the transmitter 23. The receiver 57 is coupled across the rails by

leads

36 and 37.

A complete detailed description of this system is set forth in my copending application for Letters Patent of the United States, Ser. No. 885,085, filed Dec. 15, 1969, for Long Length Track Circuit, and assigned to the assignee of the present invention. Accordingly, only a brief description of the system will be set forth hereafter. The system described will establish the environment in which the repeater to be described more fully will operate.

Without going into the details of the transmitter 23, suffice it to say that a pulse of the type shown above the rail 11, that is pulse 73, will be generated by the transmitter and will be impressed on the

rails

11 and 12 over the

leads

38 and 39 from the transmitter 23. These pulses of the type 73 will pass down along the rails and, as can be seen from the showing immediately above the rail II, the pulse 73 becomes attenuated as it passes along the rail and this factor of attenuation is illustrated by the pulses 74 and 75 which are shown to be decreasing in size due to the attenuation brought about by impedance present in the rails.

Coupled across the

rails

11 and 12 are a plurality of capaci-v tors referred to hereinbefore, These capacitors improve the transmission of the pulses along the rails and their position and number are subject of my above-referred to copending application. Specific details of the positioning of these unreferenced capacitors will not be made here but reference is made to this copending application for these details.

There is coupled across the rails a repeater 22 connected to the rails ll and 12 by

leads

13 and 14. it will be seen as one passes from the left-hand side decreasing this drawing toward the right that, in addition to repeater 22, there are a plurality of repeaters designated by the general term R electrically coupled across the rails and each positioned such that it will receive an attenuated pulse, such as the pulse 75. Repeater 22,

for example, will in turn amplify and retransmit the repeated pulse 76 to the

rails

11 and 12. The repeated pulse 76 will in turn experience attenuation as is graphically illlustrated by the pulses 77 and 78 which illustrate a decreasing amplitude of the pulse due to attenuation.

For the practice of this invention the selection of narrow pulses has been made in order that these narrow pulses permit the filtering out of low frequency noise which is believed to predominate in track circuits in general. In determining the positioning of the repeaters, as well as the function that they must perform, it should be recognized that the propagation time of pulses along loaded track is approximately 0.l4 millisecond per thousand feet with a maximum attenuation at 10,000 feet for an 0.8 millisecond pulse of about 22 db. and a minimum attenuation, for example in dry weather, of 3.7 db. Accordingly, if repeaters are connected every 10,000 feet in wet weather, the output of each repeater will be attenuated by 22 db. before it reaches the input of the next repeater. In this case, if one is referring to repeater 22, then the next repeater to receive a repeated pulse would be the next repeater R positioned to the right of repeater 22. If one were to allow a margin of 3 db. for each repeater, then the repeater should be able to be triggered by an input signal 25 db. below its own output signal. in the system being described here, each repeater must be absolutely dead, that is, unable to react, when the input pulse for some interval after it has fired. This is because once the repeater, for example repeater 22, has repeated 2. pulse for some interval after it has fired. This is because once the repeater, for example, repeater 22, has repeated a pulse and delivered it into the

rails

11 and 12, this repeated pulse will go in both directions along the

rails

11 and 12, and when the next adjacent repeater R repeats a received pulse, this repeated pulse will pass both to the right and left along the electrically

continuous rails

11 and 12. In the event that the repeater 22 is not dead, that is, unable to respond to a repeated signal from repeater R just noted, then the repeater 22 would produce a signal and the obvious confusion of signals that would appear throughout the length of the track circuit becomes a very large problem. Accordingly, one must calculate the interval these repeaters must be dead. ln calculating this interval of dead time one must look at the problem in dry weather conditions where typically the attenuation is 3.7 db. for 10,000 feet of rail. Therefore, in 80,000 feet, approximately 15 miles, a signal would suffer a minimum attenuation of 30 db. This would be db. below the specified input noted above with reference to the sensitivity needed at each receiver. And, therefore, a repeater cannot directly trigger another repeater 80,000 feet away but it can trigger intervening repeaters during dry weather, as has been noted earlier.

if a pulse is transmitted at time T=0 and this pulse comes from the transmitter 23, the first repeater 22, which is for example 10,000 feet away, will receive an attenuated pulse at T=l.4 milliseconds, and then this repeater 22 will fire. In a similar fashion, at T=2.8 milliseconds the first repeater pulse will arrive back at the transmitter 23 and simultaneously trigger the second repeater R referred to above. This process goes on and on so that a new pulse appears at 2.8 millisecond intervals. The first repeater pulse will travel 80,000 feet down the track in 11.2 milliseconds, triggering the ninth repeater. This ninth repeater pulse will travel 80,000 feet back to the repeater 22 in 1 1.2 millisecond.

By the computations of the preceding paragraph this signal will always be too weak to retrigger the first repeater. Each repeater must therefore be dead for approximately 22.4 milliseconds after it is fired. In practice there is allowed a substantial margin and the time is set at 30 milliseconds.

This repeater system is especially unique in that it is easy to troubleshoot and maintain. in dry weather one would merely connect at the transmitter end of the track circuit an oscilloscope. One would then see the transmitter pulse, which in this instance would be approximately volts, followed by the first repeater pulse, which would be about 6.7 volts, followed by the second, etc. This is a string or collection of pulses each one two-thirds the size of its predecessor. if one repeater has failed or has a dead battery, then this pulse of the collection will be missing. One thus can do the maintaining of the track circuit system employing the repeaters embodying the invention during dry weather based on oscilloscope measurements at the ends of the track circuit. It is important to note that the repeaters, which form the subject of this invention and which will be described in detail hereafter, do not load down the rails should one of them fail, for if any one of the repeaters fails, it neither subtracts from nor adds to energy on the rails. Accordingly, if the ballast resistance is not too low, this signal will leapfrog a dead repeater and trigger the next repeater down the line.

The repeater must meet certain basic requirements consistent with the requirements that are demanded by the Association of American Railroads. Accordingly, the repeater must be fail-safe in addition to providing the above-noted 30 millisecond dead time. And the fail-safeness requires that the repeater must not fire spontaneously or, in the other hand, the internal gain of the repeater must not increase so that weaker signals might trigger the repeater. Also, the dead time must never get shorter or decrease to zero. in concluding, the output pulse of the repeater must never increase in amplitude.

With these basic understandings of the positioning of the repeaters along the track, a study may now be made of the details of the repeater which embodies the invention. The most elementary system employing the repeater of this invention would involve a single transmitter and receiver, which would allow a track circuit of a very great length to be present. ln this environment the transmitter would place a pulse into the track which would be repeated down the length of the track circuit to a receiver many miles away. Accordingly, should a train enter the length of track thereby shunting the rails, the pulses would not reach the receiver and there would be an indication of track occupancy.

Reference is now made to the drawing wherein there is illustrated a circuit diagram of a pulse repeater of the type referred to above. The repeater to be described will have a dead time or time during which no pulse may trigger the repeater. The duration of the dead time and how it is accomplished will now be set forth. As can be seen in the drawing, the repeater 22, shown in dotted outline, is electrically connected across the

rails

11 and 12 by leads l3 and 14. A negative going attenuated pulse 75, described earlier, appears on lead 13 from rail 11 and enters the pulse repeater 22. The negative going pulse 75 travels along lead 131; through a current limiting resistor 16. The pulse 75 continues along lead 17 until it enters a stepup transformer 18, shown in dotted outline, when it undergoes a inversion and appears on base lead 19 as a positive going pulse, as is shown immediately above lead 19. Connected to base lead 19 is a diode 25 electrically connected by lead 24 to lead 19 and to ground by lead 26. This diode provides protection to the transistor Q1 by affording an electrical path to ground in the event there are excessive pulse of reverse polarity voltage or voltage surges due to external sources which may induce in the

rails

11 and 12 undesirable transients.

The transistor O1 is connected to a positive battery voltage source 48 and to ground. The collector 27 of the transistor 01 is electrically connected to positive battery source 48 via lead 30, resistor 31, lead 32, primary winding 33 of transformer 35, and leads 34, 47. The emitter 28 of transistor O1 is connected to ground by lead 29. Before transistor Q1 conducts, the voltage at the collector 27 is at a positive level. The appearance of the positive pulse on base lead 19 causes the transistor 01 to conduct and the output across Q1 will follow the pulse on the base lead 19, thereby inducing in the primary winding 33 of the transformer 35 a negative-going pulse which will in turn induce in winding 40 of the transformer 35 a negative-going pulse which will appear on lead 41. Accordingly, signals are standardized by transistor 01, which delivers a current pulse of constant size for all inputs above threshold. The transistor Q2 has its emitter 44 electrically connected to the positive battery source 48 via lead 46. The collector 45 of transistor Q2 is electrically connected to ground via lead 49, transformer winding 50, leads 51, 52, and resistor 53. As was noted earlier there is a negative-going pulse present on lead 41,

which pulse will pass through the diode 42 to base lead 43 of 5 the transmitter Q2. This negative-going pulse will trigger the transistor 02 into conduction. As soon as transistor 02 begins to conduct, an increased voltage will appear in transformer winding 50 which will, through regenerative feedback, cause the negative-going condition on lead 41 and base lead 43 to continue in a negative direction, thereby keeping the transistor Q2 conducting.

The transistor 02 and transformer 35 form a blocking oscillator which when triggered with transistor Q2 conducting will remain conducting for the time required to saturate the transformer 35. The selection of the transformer of course will be determined by the amount of dead time desired. in the instant application of the invention this would be 30 milliseconds. During the conducting period of the blocking oscillator it cannot be retriggered, thus providing the dead time. It should be noted that when transistor 02 turns off, the voltage on the lead 41 rises well above the potential of the positive battery supply 48. The series diode 42 thereby protects the base emitter junction of transistor 02. The series diode 42 is preferred to a shunt diode approach because if a shunting diode were employed, while dead time might be increased by as much as l0 milliseconds due to discharge of the transformer 35 through the shunting diode, should the shunting diode fall off or become disconnected then the dead time would suddenly decrease by milliseconds. This would result in an unsafe condition especially if the 10 millisecond loss cut into the desired dead time required under dry ballast conditions.

The output from the blocking oscillator which is comprised of transformer 35 and transistor 02 is a positive-going square wave appearing on lead 51. Electrically coupled to lead 51 is a differentiator which is comprised on the lead 54, capacitor 55, gate lead 56, and resistor 60 which is connected to ground. The leading edge of the square wave above noted is differentiated which results in a positive-going spike which will be employed to trigger the silicon controlled rectifier 59. A diode 61 is electrically connected to gate lead 56 and provides a similar protective function for silicon controlled rectifier 59 as was described with reference to diode and transistor Q1. The diode 61 also assures an electrical path to ground when the trailing edge of the square wave pulse from the blocking oscillator is differentiated to produce a negative going spike. The silicon-controlled rectifier 59 is electrically connected in a circuit between the positive battery terminal 48 and ground via lead 64, resistor 65, leads 66, 67, the silicon controlled recrifier 59, and finally to ground. When the spike pulse due to the differentiation of the leading edge of the square wave output from transistor Q2 of the blocking oscillator appears on gate lead 56, this triggers silicon-controlled rectifier 59 into conducting. The silicon-controlled rectifier 59 once conducting will remain conducting until the voltage bias between the anode and cathode of the silicon-controlled rectifier falls below zero, at which time the silicon-controlled rectifier 59 will cease to conduct.

At the instant the silicon-controlled rectifier 59 starts to conduct there will be present on capacitor 68, of a stored signal circuit, a charge due to the battery potential on terminal 48 electrically connected thereto by lead 64, resistor 65, and lead 66. The capacitor will dump its charge through the silicon-controlled rectifier 59 to ground and in so doing, because of the electrical connection of the capacitor 68 by lead 69, inductor 70, and lead 71 to transformer 72, induce in the secondary winding of the transformer 72 an output pulse which approximates a half sine wave which will be carried by lead 13a and thence over lead 13 to rail 11 where it will appear as repeater pulse 76.

lt should be kept in mind that the combination of the capacitor 68, the inductor 70, and the transformer 72 plays three important simultaneous functions. The first of these functions is the assurance that the silicon-controlled rectifier 59 will experience a reverse polarity, thus insuring that the silicon-controlled rectifier 59 will be turned off. In addition, there is established the size of the output current so that the circuit behavior is changed very little by a sort circuit across the output. This is accomplished because the current through the inductor 70 and capacitor 68 is oscillatory, so that the voltage across capacitor 68 will run down through zero during the pulse and then overshoot by a small amount, c.g., a few volts. While this arrangement of an inductor and capacitor in an oscillating circuit may be considered to be inefficient from the standpoint that a portion of the available pulse stored by the capacitor is used up internally in the capacitor and inductor, the highly desirable feature of being able to short the output without damage to the circuit is present. This is because the current is almost wholly determined by the inductor 70. ln the railway environment in which this invention is to be employed, the problem of a possible short between the turns of the inductor, which would result in a very large output pulse which is unsafe, is avoided because the inductor 70 is made from a few turns of heavy, well-insulated wire wound on a core and a nonshorting can.

While the invention has been shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that other modifications may be made therein without departing from the spirit and scope of the invention.

Having thus described my invention, what I claim is:

1. A fail-safe signal repeater for receiving and repeating a signal from and to an electrically continuous rail, said signal repeater including,

a. input means electrically coupled to said electrically continuous rail to receive said signal to be repeated,

b. Signal repeating output means electrically coupled to said electrically continuous rail to provide said repeated signal,

c. control means electrically coupled respectively to said input means and said signal-repeating output means to thereby initiate said repeated signal while simultaneously providing a period of nonresponse for said repeater for a predetermined period of time after said signal is received and repeated.

2. The fail-safe signal repeater of claim 1 wherein said input means includes an amplifier, transformer coupled to said rail to receive and amplify said signal form said rail, said amplifier having an output electrically coupled to said control means.

3. The fail-safe signal repeater of claim 2 wherein said con trol means includes a triggered block oscillator electrically coupled to said amplifier output to be triggered thereby,

said triggered blocking oscillator having an output signal electrically coupled to said signal-repeating output means of a length equal to said predetermined period of time, said blocking oscillator output signal providing said period of nonresponse for said signal repeater.

4. The fail-safe signal repeater of claim 3 wherein said signal-repeating output means includes a stored signal energy means as well as a switch circuit, said switch circuit electrically coupled respectively to said block oscillator output and said stored signal energy means, said switch circuit responsive to the appearance of said blocking oscillator output signal to allow said stored signal energy to be delivered to said electrically continuous rail as said repeater pulse,

5. The fail-safe signal repeater of claim 4 wherein said switch circuit includes a differentiator electrically coupled to said blocking oscillator output as well as a gate, said gate electrically coupled to said stored signal energy means and controlled by the initial difierentiation of said blocking oscillator output signal to thereby allow the release of said stored signal energy to said electrically continuous rail.

6. The fail-safe signal repeater of claim 5 wherein said gate is a silicon-controlled rectifier with its gate electrically coupled to said differentiator and its anode electrically connected to said stored signal energy means.

7. The fail-safe signal repeater of claim 6 wherein said stored signal energy means is a series-connected capacitor and inductor with said capacitor electrically coupled to said anode of said silicon-controlled rectifier and with said inductor electrically coupled to said rails to thereby permit the shorting of the repeater without the risk of inducing a very large pulse to said rails and accordingly insure fail-safe operation.

8. The fail-safe signal repeater of claim 7 wherein said signal to be repeated is a pulse.

9. The fail-safe signal repeater of claim 3 wherein said triggered blocking oscillator is transformer coupled to said amplifier output,

said transformer couple including a primary, a saturable core, a first secondary, and a second secondary,

said triggered block oscillator further including a transistor with its base electrically coupled through a series diode to said first secondary and the collector of said transistor electrically coupled to said second secondary,

said period of nonresponse occurring during a period of conduction of said transistor when said amplifier output initially appears and said saturable core is saturating,

said series diode providing protection against excessive voltage surges caused by the collapse of the field surrounding said saturable core at the end of said period in which said saturable core is saturating, said series diode thereby insuring that said period of nonresponse is always of the same time duration.

10, A fail-safe signal repeater for receiving and repeating a signal from and to an electrically continuous rail, said signal repeater including,

a. input means electrically coupled to said electrically continuous rail to receive said signal to be repeated, said input means having an amplifier, transformer coupled to said rail to receive and amplify said signal from said rail, said amplifier having an output electrically coupled to a control means, said control means electrically coupled to said input means and a signal-repeating output means, said control means having a triggered blocking oscillator electrically coupled to said amplifier output to be triggered thereby, said triggered blocking oscillator having an output signal electrically coupled to said signal repeating output means, said output signal having a length equal to a predetermined period of time, said blocking oscillator output signal providing a period of nonresponse for said signal repeater, c. said signal repeating output means having a stored signal energy means as well as a switch circuit, said switch circuit electrically coupled respectively to said blocking oscillator output and said stored signal energy means, said switch circuit responsive to the appearance of said blocking oscillator output signal to allow said stored signal energy to be delivered to said electrically continuous rail as said repeated pulse.

Claims

1. A fail-safe signal repeater for receiving and repeating a signal from and to an electrically continuous rail, said signal repeater including, a. input means electrically coupled to said electrically continuous rail to receive said signal to be repeated, b. Signal repeating output means electrically coupled to said electrically continuous rail to provide said repeated signal, c. control means electrically coupled respectively to said input means and said signal-repeating output means to thereby initiate said repeated signal while simultaneously providing a period of nonresponse for said repeater for a predetermined period of time after said signal is received and repeated.

2. The fail-safe signal repeater of claim 1 wherein said input means includes an amplifier, transformer coupled to said rail to receive and amplify said signal from said rail, said amplifier having an output electrically coupled to said control means.

3. The fail-safe signal repeater of claim 2 wherein said control means includes a triggered block oscillator electrically coupled to said amplifier output to be triggered thereby, said triggered blocking oscillator having an output signal electrically coupled to said signal-repeating output means of a length equal to said predetermined period of time, said blocking oscillator output signal providing said period of nonresponse for said signal repeater.

4. The fail-safe signal repeater of claim 3 wherein said signal-repeating output means includes a stored signal energy means as well as a switch circuit, said switch circuit electrically coupled respectively to said block oscillator output and said stored signal energy means, said switch circuit responsive to the appearance of said blocking oscillator output signal to allow said stored signal energy to be delivered to said electrically continuous rail as said repeater pulse.

5. The fail-safe signal repeater of claim 4 wherein said switch circuit includes a differentiator electrically coupled to said blocking oscillator output as well as a gate, said gate electrically coupled to said stored signal energy means and controlled by the initial differentiation of said blocking oscillator output signal to thereby allow the release of said stored signal energy to said electrically continuous rail.

9. The fail-safe signal repeater of claim 3 wherein said triggered blocking oscillator is transformer coupled to said amplifier output, said transformer couple including a primary, a saturable core, a first secondary, and a second secondary, said triggered block oscillator further including a transistor with its base electrically coupled through a series diode to said first secondary and the collector of said transistor electrically coupled to said second secondary, said period of nonresponse occurring during a period of conduction of said transistor when said amplifier output initially appears and said saturable core is saturating, said series diode providing protection against excessive voltage surges caused by the collapse of the field surrounding said saturable core at the end of said period in which said saturable core is saturating, said series diode thereby insuring that said period of nonresponse is always of the same time duration.

10. A fail-safe signal repeater for receiving and repeating a signal from and to an electrically continuous rail, said signal repeater including, a. input means electrically coupled to said electrically continuous rail to receive said signal to be repeated, said input means having an amplifier, transformer coupled to said rail to receive and amplify said signal from said rail, said amplifier having an output electrically coupled to a control means, b. said control means electrically coupled to said input means and a signal-repeating output means, said control means having a triggered blocking oscillator electrically coupled to said amplifier output to be triggered thereby, said triggered blocking oscillator having an output signal electrically coupled to said signal repeating output means, said output signal having a length equal to a predetermined period of time, said blocking oscillator output signal providing a period of nonresponse for said signal repeater, c. said signal repeating output means having a stored signal energy means as well as a switch circuit, said switch circuit electrically coupled respectively to said blocking oscillator output and said stored signal energy means, said switch circuit responsive to the appearance of said blocking oscillator output signal to allow said stored signal energy to be delivered to said electrically continuous rail as said repeated pulse.