Track circuit equipment is provided in which the transmitter of each track circuit uniquely identifies its transmissions by impressing an associated data word (38) onto its transmission using frequency shift keying. The track circuit receivers are so arranged that in order to output a track clear signal, a receiver must first have detected on incoming signals a data word (38) identifying the transmitter belonging to the same track circuit as the receiver. Each data word (38) is included in a message structure that also provides a start sequence (35) and a parity section (39). The data word (38) includes respective bit groups providing longitudinal and lateral location information on the related transmitter. Each data word (38) may also include a bit group providing for an auxiliary signalling function such as track-to-train signalling.
