A transponder circuit 1 comprises a (track mounted) loop 4 for inductively coupling with a (train mounted) interrogator 2 to transmit a binary coded message. The loop 4 forms part of a tuned circuit which is tuned to a first, resonant frequency Fc and which is detunable to a second, lower frequency Fc/2 whereby a binary code can be created utilising said two frequencies. The first resonant frequency Fc is determined by capacitor C2 across the loop 4 and is halved by the switching in of a second capacitor C3 by FET.TR. Transistor TR is controlled by an EEPROM 6 to generate a binary code in which '1' and a '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' (Fig. 2), resulting in data invariant energy transfer. The system is suitable for transmitting speed and other track information to a passing train. <IMAGE>
TRACK TO TRAIN COMMUNICATIONS SYSTEMS