Abstract: A railway signal system detects a train approaching the system tie points to a railroad track by transmitting in the track a modulated AC carrier wave signal, which is attenuated by the variable shunt effect of an approaching train, and monitoring the track signal in a receiver that responds to signal variations caused by such an approaching train to drop a signal relay, which operates a gate, flasher, or the like at a grade crossing. The track signal is normally automatically regulated to be within a voltage window, and a continuous system latch up feature precludes pick up of the signal relay upon occurrence of a broken rail or high track signal, for example, caused by a very high impedance ballast, until manually reset. A time delay circuit delays pick up of the signal relay after motion has been first detected, and a bypass circuit for a low track signal detector allows system operation even after an unexpected shunt occurs on a monitored section of track.

Abstract: A first transmitter-receiver movement detector system operable at a first frequency detects a train approaching a first grade crossing or other track location in response to train approach speed and distance, and a repeater receiver movement detector device also operable at such first frequency but located at a subsequent grade crossing or location, which is relatively proximate the first, is capable of looking through the first crossing even under unfavorable ballast conditions thereat to detect an approaching train with respect to its speed and distance from the second crossing. The output of the repeater receiver and that of a further transmitter-receiver movement detector system operable at a different frequency from the first are combined in a logic circuit to provide safe, minimum signal down time at the subsequent location.