Patent GB-836434-A (GB)
Title: Electronic track circuit for railroads
IPC: B61L21/06
Filed: 1956-10-03
Abstract: 836,434. Railway track circuits. GENERAL RAILWAY SIGNAL CO. Oct. 3, 1956 [Oct. 5, 1955], No. 30149/56. Class 105. An electronic railway track circuit which may be used in addition to the usual track circuit comprises an electronic, e.g. transistor, oscillator 10, e.g. of a relatively high audio-frequency, say 10 kc/s., feeding, through a coupling circuit 40, 41 including the track rails so as to be completed only when the rails are shunted by a train, into a receiver responsive to the oscillator output frequency to give a distinctive output. The coupling circuit includes a tuning capacitor 42 in series with the output circuit 40 of the oscillator and input circuit 41 of the receiver 33 and is such as to couple a maximum amount of energy to the receiver when the rails are shunted at a preselected distance from the point of connection FP of the coupling circuit with the rails. A tuned circuit 36, 37 to the transmitter frequency across the secondary winding of the receiver input transformer T3 by-passes all other frequencies, the secondary winding feeding, through a full-wave rectifier 38 an electromagnetic relay TR having connected thereacross an adjustable resistor 39 for varying the sensitivity of the relay, and is adjusted to reduce the input impedance thereof so that an impedance change across the rails due to a shunt, is of maximum effectiveness in operating the relay. The oscillator comprises a transistor oscillation stage 11 and a transistor push-pull amplifier stage consisting of transistors 22-25 of the PNP junction type, feeding into the output transformer T2, the secondary winding of which is in the coupling circuit. As applied to a points lock control the circuit is fail-safe in that any circuit fault results in inability to pickup the track relay TR so that non-occupancy of the track is indicated. As used in an A.P.B. signalling system the points lock mechanism is effective when locked to prevent operation of the points by the switch machine SM manually or automatically, from its normal condition. A lock magnet LM is operated by operation of the track relay TR closing its contact 67, on a train effectively shunting the track rails to unlock the points and permit operation of the switch machine, assuming a padlock has previously been removed from the points lock mechanism resulting in the operation of the contacts 62, together with contacts 53 and 65. The contacts 53 on opening de-energize the lock normal points repeater relay LNWP which is normally energized over contacts 54, 55 in the right-hand position for the points in the normal position. On relay LNWP dropping away, front contacts 58 open to remove energy normally connected to the wire 2HD to the right of contact 58 of the wayside block end controlling signal line, and front contacts 59 open to remove energy normally connected to the left of the other wire 5HD of the line, and under these circumstances the signals 2, 5 are caused to display their stop aspects. For the movement of a train from the siding to the main line, with no train on the line, relays WF and EF pull up over back contacts 58, 59 and over their front contacts 60, 61 and front contact 62 operate the lock magnet LM so that the points machine SM may be fully operated to the reverse position. Should there be a train on the line between signals 2 and 5 either relay WF or EF fails to pick up when relay LNWP releases and the time relav TER is energized over back contact 60 or over front contact 60 and back contact 61, back contact 63 and back contact 64 of relay TR and front contact 65 and after a preselected time interval, of the order of 5 minutes or more, the relay TER operates, the interval being such that a train which has entered the block prior to the signals 2 and 5 operating to stop aspect, would have sufficient time to reach the location of the sidings and having passed, the operation of the points machine SM to the reverse position may be safely made. On operation, relay TER energizes the relay LM over its front contact 66. For the movement of a train from the main line to the siding, the auxiliary track circuit provides an indication that the line is occupied immediately in approach of the points and enables the time interval of relay TER to be eliminated, the energizing circuit for the lock magnet being then over contacts 67 and the points can be reversed immediately. A spurious shunt in the vicinity of the point FP can cause relay TR to pick up thereby releasing the switch lock mechanism over back contacts 70, 71 and causing signals 2 and 5 to display the stop aspect.