Abstract: A train approach detector uses frequencies in the audio spectrum to activate grade crossing protection devices in advance of and during the presence of railroad trains at grade crossings. The detector is completely solid state, and includes one transmitter and one receiver each connected to the tracks so that transmitted and received track signals, which are partially pulse modulated for optimum filtering and signal coupling and minimum third harmonic radiation, appear on opposite sides of the crossing. The section of track between the transmitter and receiver is designated as a crossing island and sections of rail outside of the island are known as approaches.