EN 50121-4 is Part 4 of the European railway electromagnetic compatibility series and specifies the emission and immunity requirements for the signalling and telecommunications apparatus installed along a railway. The equipment covered includes the interlocking and control systems, the track circuits and axle counters, the balises and the radio and cable transmission systems on which the safe operation of the railway depends, and it is installed in an environment that is both electrically severe and safety critical. The apparatus must tolerate the fields and the conducted disturbance produced by passing trains, by the traction current returning through the running rails and the ground, and by the traction supply itself, without any change in the behaviour of a safety function. The standard defines the emission limits that protect other equipment, including the radio services used by the railway, and the immunity requirements with the levels applicable to track-side and to signalling room installations, together with the test conditions and the performance criteria appropriate to signalling equipment.