Abstract: A communication system for a railway train has a proving circuit for proving conditions of the railway track ahead of the train. The proving circuit comprises a transponder associated with at least one rail of the track to form a rail circuit therewith. The response of the transponder to an interrogater on the train is dependent upon the condition of the rail circuit.

Abstract: A center-fed track circuit arrangement for an electrified section of railroad track. A first and a second remotely located insulated joint defining the limits of the track circuit. A first impedance bond connected across the rails of the railroad track in the vicinity of the first remotely located insulated joint. A second impedance bond connected across the rails of the railroad track in the vicinity of the second remotely located insulated joint. A third impedance bond connected across the rails of the railroad tracks intermediate the first and second remotely located insulated joints. Each of the first and second impedance bonds includes a center tapped winding for accommodating propulsion current. A pair of tuned windings for accommodating at least two train detection signals having different frequencies. An untuned winding for exhibiting a low impedance at the frequency of a cab signal so as to mitigate leakage current across a broken-down insulated joint.

Abstract: An improved audio frequency track circuit system utilizes a synchronous train detection arrangement and reduces the number of fixed code rate modulated carrier signals. As few as two carrier signals are alternately applied at discrete points along a pair of jointless track rails to define the transmitter ends of the track sections, with complementary receivers defining the opposite ends. The two carrier signals are coded at one of two phase angles which are 90.degree. out-of-phase so that, when assigned, the nearest possible interfering signal is 90.degree. out-of-phase and is rejected thereby. Like carrier frequency transmitter/receiver arrangements are disposed on opposite sides of an insulated joint but are coded 180.degree. out-of-phase so that a breakdown of the insulated joint is detected and the false code signal is rejected.

Abstract: A self-powered receiver including a series resonant circuit tuned to the carrier frequency of a modulated carrier signal. A demodulator for detecting the envelope which switches a switching transistor ON during the mark portions of the modulator carrier signals and switches the switching transistor OFF during the space portions. A parallel resonant circuit is driven by the switching transistor to produce a.c. modulations to be coupled to a full-wave rectifier. The rectified voltage of the rectifier is used to energize an electromagnetic relay.