Abstract: There is disclosed a double rail track circuit for use within a railroad block or each cut section of a block. Adjacent cut sections, or blocks, are separated by insulated rail joints. A transmission line having terminals accessible at each joint is provided. The transmission line has impressed thereon a modulated carrier signal to provide a reference signal at the modulation frequency. The reference signal derived from the carrier is coupled to one end of each block, or cut section, in such a manner that the signals of adjacent blocks, or cut sections, are out of phase with respect to each other. A synchronous detector is coupled between the transmission line and the rails at the other end of the block, or cut section, to compare the phase of the signal at the other end, with the anticipated signal phase. Band-pass filters may be used between the rails and the synchronous detector and between the transmission line and the synchronous detector and which do not pass common frequencies.