At each end of a track stretch, a relay chain driven by a code transmitter at times connects across the rails in repeated sequences a polar, code following track relay, a track battery, and a rail shunt path. A similar sequence of rail connections at each intermediate repeater location is produced by a front contact repeater and a decoding repeater of the corresponding track relay. Each repeater relay has a selected slow release to produce the sequence following each response by the track relay to a received energy pulse. The pulse following operation of each end location track relay is decoded by a repeater relay which connects the track relay continuously across the rails and inhibits relay chain operation. A shorter relay chain at one end prevents scrambled code and provides a dominant pulse transmitter with no train occupancy and no entering signal cleared. Each repeater location transmits in either direction and requires no insulated joints to separate adjoining sections since track battery connections are of alternately opposite polarity from location to location. The track rail shunt applied following pulse transmission at each location prevents response by the associated track relay to the subsequent pulse transmitted from the adjacent advance location and thus maintains pulse direction. To prevent false operation, each track relay is rail connected with polarity opposite to the same location track battery. Clearing an entering signal sets up pulse transmission from the other end.