An active loop conductor (LSI) having a sensing region (A1) is positioned adjacent the railway track. A vehicle detector (FDI) is connected to the loop conductor and, in its ground state when it is not activated by the presence of a vehicle, delivers a 'vacant' code at its output. This in turn controls an output relay (D1). Alternatively it may feed an electronic circuit element such as a semiconductor device or a transfluxor. As soon as a vehicle (F) enters the sensing region of the loop conductor, the vehicle detector (FDI) operates in known manner by inductive damping or detuning and changes the 'vacant' code into a 'vehicle present' code. The sensing region of the loop conductor is larger than the smallest vehicle to be detected and smaller than the largest.
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