Abstract: A railway signalling system for the detection of a train within a defined section of track by means of track circuit apparatus utilizes the rails within the section as part of the track circuit. The rails are electrically shunted by the wheels and axles of a railway vehicle of the train in the section. The presence of a train is detected by detecting the change in the shunt impedance between the rails of the track circuit when a train enters the section. To improve the reliability of the track circuit a shunt assist circuit is provided. This shunt assist circuit comprises an inductive loop aerial provided on the railway vehicle so that it is closely coupled, inductively, with the rails, whereby when the loop aerial is energized from an alternating source, a current is induced in the wheel-rail-axle circuit.

Abstract: A transponder circuit comprises a loop (4) for inductively coupling with an interrogator (2) to transmit a binary coded message to the interrogator (2); the loop (4) forms part of a tuned circuit which is tuned to a first resonant frequency Fc and which is detunable, by an associated modulator, to a second lower frequency Fc/2 whereby a binary code can be created utilizing the two frequencies; and advantageously, a binary "1" and a binary "0" each comprise two cycles of Fc and one cycle of Fc/2 with a binary "0" having the reverse sequence of the frequencies to a binary "1".