Abstract: Alternating current track circuits are provided for the several adjoining track sections of a stretch of railway track, each of the track circuits comprising an alternating current code transmitter and an alternating current code receiver connected to track rails at opposite ends of the associated section. Circuit apparatus including an alternating current line circuit extending along the stretch of railway track is provided for normally communicating alternating current from the line circuit through rails of the associated track sections, the alternating current being modulated at a selected rate code and current in any two adjoining track sections being of opposite instantaneous polarity for broken down joint protection. Phase sensing apparatus is provided at a receiving end of each track section including a comparator for comparing the rate code modulated alternating current communicated through the track rails with a steady reference source connected to the line circuit.