Abstract: An automatic vehicle protection system, e.g. for a railway vehicle, in which continuous contact is maintained with a train via transmissions carried by the normal track circuit signals. Information relayed to the train through these transmissions may concern speed limits, forward section occupancy and so on which can replace normal track side visual signalling systems as a means of controlling trains. It is, therefore of utmost importance to ensure that a train receives the correct transmission, which it is intended to receive, and not those meant for other trains. By arranging for track circuit signal freqencies in predetermined sequences and providing the train with means for determining whether or not the sequence of frequencies it encounters as it traverses a line the safety of the train, and the integrity of the system may be continuously proved.