Abstract: A digital signal is used for railway track circuit signalling. A digital signal comprises a pseudo-random binary sequence selected from a set of such sequences. The set is chosen to give the lowest possible cross-correlation between different sequences, to minimize the risk of false identification. Gold codes are suitable. One of the sequences indicates the absence of a train from the track section and this code is generated until an indication is given by a train detector that a train is present. Generation of the train absence signal is then inhibited, and other sequences are generated providing information to the train. The generated sequences are received and coded in a fail-safe decoder.