An island circuit detects the presence of a train on a crossing island, and, in response, de-energizes a normally energized relay to activate a warning. A random signature signal is generated and transmitted through the rails from a transmitter to a receiver connected to the rails on either side of the island. If a train is present within the island, the wheels and axles form an electrical shunt, preventing the receiver from detecting the transmitted signal. The circuit employs solid state micro-electronic components and ancillary components, and embedded software for controlling and monitoring the circuit functions. The random signature signal is characterized by a relatively high voltage level but a low duty cycle on the order of ten percent, and has three variables: (1) frequency; (2) duration time of the pulse, or burst, of the signal; and (3) the delay between successive bursts. Each of the three variables randomly assumes any one of eight different discrete values. Each time a signal is received, the detector checks to determine whether the received signal matches the transmitted signature signal. If three consecutive transmitted and received signals do not match, the island warning device is activated, and then five consecutive matched signals are required to reset the system and cancel the island warning. This feature allows the island circuit to discriminate against signals from other island circuits or recover from a temporary malfunction, and return to normal operation.