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.