When a prior art track circuit for monitoring an electric signal supplied to a rail via a transmitter and a receiver connected to the rail for detecting a presence of a train using a phenomenon in which the reception level of a signal drops when axles of a train electrically short two rails together and sending a train control signal to the train is applied to a dual gauge track circuit in which two types of trains having different gauges share a rail, mutual induction between rails occurs which is a problem specific to the dual gauge track circuit, and the amount of attenuation of the train detection signal when a train is present on a track drops. The present invention prevents the mutual induction between rails and suppresses the drop of the amount of attenuation of the train detection signal when a train is present on a track by providing a train detector for recognizing two types of trains and detecting the presence of trains on the track via transmitters and receivers respectively connected to three rails, including filters for passing relevant signals and increasing the impedance of signals of other signal bands disposed on input stages of the respective receivers.