An overlay track circuit is used to transmit information through a block of railroad tracks. In one embodiment, one of a plurality of codes assigned to a receiver/transmitter pair are used to represent each information segment (which may be one or more bits) of an information signal. In another embodiment, bits of the information signal are transmitted between periodic repetitions of a code associated with the transmitter/receiver pair. The modulation may be performed using a frequency shift key technique. Track circuits may be connected to relay information between adjacent blocks of track.