Abstract: A railroad highway crossing warning system including pickup coils for sensing current flowing in the track rails of an approach zone, a circuit for sensing the voltage across the track rails, band-pass filter circuits for filtering the voltage and current derived from the track rails, impedance calculator circuits for calculating the track impedance, phase detector circuits for detecting the phase angle between the voltage and current, a multiplier circuit for linearizing the track impedance by multiplying the track impedance by a function of the phase angle, a data sampler circuit for sampling the linearized track impedance at given periodic intervals, a motion detector and crossing predictor circuit for detecting the motion of a train and for predicting the time of arrival at the crossing by comparing the predicted time of arrival with an advanced warning time, and a logic gate circuit for activating the warning apparatus at the crossing when the predicted time of arrival is less than the advanced warning t

Abstract: A highway crossing warning system for monitoring the motion and predicting the time of arrival of an approaching train at the highway crossing and for detecting the presence of a broken rail in the approach zone by feeding dual frequency signals into the track rails and measuring the track impedances at the two frequencies and the phase angle of the lower of the two frequencies.