Railway Cab Signal Transmitter

Automatic block signal systems using wayside signals provide the primary control for railway vehicle operation, but it is often desirable to have additional, on-board signal displays. On-board, or cab signals, are particularly useful where rain, fog, or other environmental conditions make it difficult to see the wayside signal aspect. In addition, cab based signal displays permit a railway vehicle operator to monitor changing track condition after the train has entered a block. Without cab signaling the train may only be permitted to proceed at a restricted speed, even if the block has been cleared.

Cab signaling is well-known and has been used for many years with a transmitter applying a signal to the rails, and a railway vehicle mounted receiver inductively sensing the coded cab signal current being fed to the track rails. Receiver coils mounted on the locomotive ahead of the leading wheels are often used to sense cab signals. The rail current between the transmitter and the leading axle is inductively sensed by the railway vehicle receiver and the appropriate signal is displayed in the vehicle cab.

When a train crosses the insulated joints at the entering end of an unoccupied track circuit, its cab signal receiver will begin to sense the coded cab signal current in the rails immediately ahead of the leading axle. As the train proceeds through the track circuit, the level of this signal gets progressively higher as the rail impedance between the signal source and the train decreases. In track circuits the rail current can be as high as 20 amperes when the train reaches the exit or leaving end, whereas the current required to energize the cab receiver may be as low as 1.3 amperes. While the rail current is being sensed in advance of the leading axle, a certain amount of the track current that carries the cab signal is shunted through the railway vehicle wheel and axle assemblies, often referred to as a train shunt. If the impedance of the train shunt is above zero, even by as little as a few hundredths of an ohm, enough cab signal rail current may bypass the train (runby current) to cause pickup of the cab signals by the receiver of a following train. This runby cab signal current can, if sufficiently large, cause a second or following train to erroneously detect the clear signal intended for the lead train. Because the rail impedance and the ballast between the trains act to reduce the level of current reaching the following train, the problem of bypass current is particularly bothersome when the following train is in relatively close proximity to the lead train. In this condition, a substantial portion of the bypass current from the lead train is available to be sensed by the following train, and is highly undesirable.

Because the runby current under a lead train is highest at the leaving end of the block, the invention provides for means to detect the approach of a train near the leaving end of the block and, when a train is detected there, to reduce the level of cab signal current applied to the rails in response to sensing such approach. Because of the increasing voltage drop across a series impedance with the track load, as the train approaches the voltage reduction on the rails causes the dropping out of a relay which in turn reduces the cab signaling voltage applied to the track circuit. The relay drop out is adjusted to sense the approaching railway vehicle at a position where runby cab signal current could exist at a level high enough to be erroneously interpreted by a following vehicle. Multiple taps on the code signal source transformer can be used to provide a reduced code signal level when said relay drops out.

FIG. 1 is a diagrammatic representation of a prior art cab signal transmitter with leading train A and following train B.

FIG. 2 is a diagram of the rail current available to two trains in close proximity within a block using a cab signal transmitter such as that shown in FIG. 1.

FIG. 3 is a presently preferred embodiment sensing load changes on the primary of the cab signal feed transformer and using both a 100 cycle cab signal and a 200 hertz track signal.

FIG. 4 is a cab signal current diagram for the two trains shown in FIG. 3.

FIG. 5 is a diagram of a presently preferred embodiment in which a 100 hertz signal is used for both the cab signal and the wayside track signal.

FIG. 1 shows a prior art railway cab signal transmitter which supplies a coded cab signal to rails 1 and 2. Rails 1 and 2 are part of a block separated from adjacent tracks by joints 3a-3d. The transmitter is attached to the rails at the leaving end of the block, which also contains impedance bond 4 shunting the rails 1 and 2. A feed transformer 5 having a secondary winding 5a connected across the rails and a primary winding 5b is also used. Connected to the primary winding 5b is a current limiting resistor 6. A CTPR or code transmitter repeater 7 has contacts 7a which alternately open and close to code the signal from the input voltage E. In this circuit CTPR and input E provide a means for generating a coded cab signal. Typically both trains A and B would have railway cab signal receivers onboard. The receivers are well-known and these devices do not form part of this invention. The onboard receivers generally sense the current in advance of the leading wheel and axle assembly on each respective train. This figure shows the trains diagrammatically; and as the expression train is often used in this specification, it is understood that the train may be a single or multi-car locomotive or transit vehicle capable of carrying freight or passengers. But, regardless of the type of vehicle, the cab signaling will usually occur at, or in advance of, the lead axles. The wheel and axle assemblies of the train provide electrical shunts between rails 1 and 2. The voltage E and the value of resistor 6 are chosen such that the preceding train A can reliably sense the cab signal upon entering the block. As train A advances toward the leaving end, it does indeed shunt an appreciable amount of the rail current, but simultaneously the rail current will increase due to the fact that the rail impedance between the leaving end and the train is reduced.

FIG. 2 shows the rail current that could be sensed by train A and train B as they move through the block. In this example the circuit parameters of the code signal transmitter of FIG. 1 have been adjusted to provide an entering end axle current of 2 amperes under minimum ballast resistance conditions of 3 ohms per thousand feet. The curves depict the current levels at infinite ballast resistance. This graph assumes that there is a constant separation between train A and train B of two hundred and fifty feet. As train A approaches the leaving end the current in the rails beneath it increases greatly. In this example 1.5 amperes has been assumed to be the minimum cab signal rail current necessary to be detected by the cab based receiver. It is clear that train A at all times can detect the cab signal. Upon entering the block, trailing train B cannot detect the cab signal because the runby coded cab signal rail current is less than 1.5 amperes. However, as train A approaches the 2500 foot distance from the leaving end sufficient rail runby current will bypass train A and be available to be sensed by train B. At this position (2500 feet) trailing train B will be able to detect the 1.5 amperes of runby cab signal. Train B in this example is behind train A by 250 feet and is erroneously able to detect a clear signal which is intended to be received only by train A. As train A is about to leave the block the cab receiver current available to train B is approximately 3.5 amperes. This undesirable condition permits train B to display in its cab the signal intended for train A. FIG. 2 also shows current in excess of 20 amps in the rails as train A reaches the cab signal transmitter at the leaving end.

As previously described with regard to bypass current cab signals, such occur more readily at the leaving or exit end of a block where the rail current can become quite high. The invention detects the load resulting in such high currents as indicative of an approaching train and adjusts the code signal to a lower value.

FIG. 3 shows rails 1 and 2 connected by an impedance bond 4 at the leaving end. Signals are applied to the rails by means of transformer 5 having secondary winding 5a connected across the rails. Cab signals from transformer 16 and wayside signals from transformer 17 can be applied to the primary winding 5b to transmit such signals to the rails. In normal operation the code transmitting repeater (CTPR) 7 periodically oscillates contacts 7b and 7c respectively to apply either the cab signal from transformer 16 or the wayside signal from transformer 17. Inductor 14 in series with the signals is used for current limiting and phasing and is a normal component of typical track circuits. In longer track circuits more inductance may be needed in the cab signaling circuit and reactor 15 can be inserted where required. When CTPR, 7, is in the down position contacts 7b and 7c connect the 120 volt 200 hertz wayside track signal transformer to the primary 5b through inductor 14. The 200 hertz track circuit signal is used for wayside controls and is not interpreted or subject to misinterpretation by trains A or B in FIG. 3. However, when CTPR moves to the upward position, the cab signal transformer 16 is now applied to the primary winding 5b of transformer 5 and via the secondary winding 5a the cab signal current is delivered to rails 1 and 2. A second set of leads come off of the transformer primary 5b and lead to the rectifying bridge 9. However, because of the operation of CTPR contact 7a, the rectifier bridge 9 is not operational except when the CTPR is in the up or cab signal position. When the CTPR is in the up position, contact 7a is closed and the primary voltage on transformer 5b is placed across the rectifier bridge 9. The rectified voltage is then applied to the series combination of resistor 12 and relay 11. If train A is far from the leaving end, or indeed if the track is unoccupied, then the voltage across 5b is high and relay 11 is caused to pickup. Relay 11 activates contact 11b to an up position which provides EFI from the cab signal transformer 16 through inductor 15 and inductor 14 to the primary of the track transformer 5. This full voltage level cab signal is then applied through the transformer 5 to rails 1 and 2. In addition to activating relay 11, the output of the rectifier bridge 9 also causes capacitor 10 to charge. Capacitor 10 is sized so as to discharge through relay 11 and prevent relay 11 from releasing during the coded periods where the input voltage to the rectifying bridge is off. This causes relay 11 to hold in its up position contact 11b connecting the full cab signal level EFI from transformer 16 to the contact 7b. As train A approaches the leaving end of the block, the rail current will increase, as previously shown. When the rail current increases to the level where a significant runby of cab signal rail current and the possibility for an erroneous signal being picked up by train B could exist, it is desired that relay 11 should drop out. This point at which relay 11 drops is controlled by variable resistor 12. If, for example, it is determined that 500 feet from the leaving end the rail current is sufficiently high that it is desired to avoid runby currents the resistor R12 can be adjusted such that when train A reaches the 500 foot position from the leaving end the voltage on the primary 5b when rectified and fed through resistor 12 will be insufficient to hold relay 11 upward and relay will drop causing contacts 11a to close and 11b to move to its lower position. At this point contact 11b will move from the EFI tap to the EF2 tap on code signal transformer 16. The EF2 tap on transformer 16 has been chosen to provide sufficient current in the rail that train A can detect the code signal at the 500 foot approach distance. In this example it is to be understood 500 feet is being used as the approach distance where relay 11 drops, it being understood that other approach distances are equally included within the scope of this invention.

Once relay 11 is caused to drop resistor 13 is placed in parallel around resistor 12 reducing the series resistance in the relay 11 circuit and making the circuit more sensitive for pickup. The pickup of relay 11 is therefore controlled by the parallel combination of resistors 12 and 13, while the dropout is controlled only by resistor 12. Typical values for resistors 11 and 12 are of the order of thousands of ohms and are adjustable depending on the specific track configurations and the relay 11 used in the circuitry. When relay 11 has dropped and the code circuit is now being supplied via contact 11b from code transformer terminal EF2 the rail current available for runby of train 1 is greatly reduced.

FIG. 4 shows a diagrammatic representation of the rail current available to trains A and B from the circuit of FIG. 3. As can be seen the current available to be sensed by the leading train A at 1800 feet from the leaving end is well above the 1.8 amps necessary to reliably activate the cab signal receiver. In addition the current under the following train B is less than the 1.5 amps minimum necessary for the cab signal receiver in the train B to be activated. Therefore, train B does not receive the clear cab signal which train A will reliably receive at the 1800 foot point. As train A progresses through the block towards the leaving end, the current under train A steadily increases until it reaches the 500 foot approach distance. Train B current (runby) has also increased but remains slightly below the 1.5 amps minimum needed to activate the cab signal on train B. At 500 feet the relay 11 in FIG. 3 senses the presence of train A at the approach distance by the high load on transformer 5 which has caused a voltage drop across both inductances 14 and 15. As a result the voltage across 5B is less than necessary to supply adequate current through relay 11 and relay 11 is caused to drop. As a result the applied voltage from code signal transformer 16 is reduced from EFI to EF2, with the resulting drop in rail current as seen at the 500 foot position on FIG. 4. As train A continues to approach the leaving end of the block the current in the rail beneath train A will continue to build until it reaches a maximum at approximately 4 amps on leaving the block. As the leading train A is shunting the leaving end of rails 1 and 2 the peak current seen by following train B is still less than the 1.5 amps necessary to activate the cab based receiver in train B. In addition during the transition and at all times train A has had at least the 1.8 amps necessary to reliably hold the cab signal. The 1.8 amps is assumed to be the minimum working value of the cab based receiver and this value has been maintained at the rails beneath train A throughout its journey.

Even after train A leaves the block the cab signal applied voltage will remain at the EF2 level if train B is within the pre-established approach distance, such as 500 feet used in this example. Because train A is no longer shunting rail current through its wheel and axle assemblies the reduced voltage EF2 available to train 2 will be sufficient to provide current under train 2 greater than the 1.5 necessary to activate its cab signal receiver. If train B is a distance further than the approach or relay drop distance, such as the 500 foot example, then the load on transformer 5 will decrease and the train sensing circuitry will not detect the train and relay 11 will pickup, thereby applying full EFI cab signal voltage to transformer winding 5a. The train sensing circuitry which detects the increased load of the rails 1 and 2 functions even when only one train is present. In this way it reduces the high currents and resulting power necessary from feed transformer 5 as trains approach the leaving end of the block.

Referring to FIG. 5 there is shown a transmitter which can be used where both cab and wayside signaling are at a frequency of 100 hertz. A current limiting resistor 19 is shown as typically used in such an application. When relay 11 is not activated or picked up, contact 11b provides EF2 output to the CTPR contacts. The reduction in signal strength while a train is being detected within the approach distance is not important to the wayside track receiver as it is desired that such signal receiver behind the approaching train display an occupied track.

Certain presently preferred embodiments have been shown in FIG. 5, and it is understood that the respective train A and train B have rail currents similar to those which have been described with regard to FIG. 4. While certain presently preferred embodiments of the invention have been described, it is understood that other embodiments are included within the scope of the following claims.