EN 50121-4 is a European Standard that applies to signalling and telecommunication apparatus installed within the railway environment. It sets limits for electromagnetic emission and immunity, ensuring performance criteria for apparatus that may otherwise cause Electro-Magnetic Interference (EMI) within or outside the railway system. This standard covers vital equipment like interlocking systems, apparatus within specified zones, and ports with specific connection lengths. It does not include safety requirements for electric shock or fault conditions. Compliance with EN 50121-4 is essential for maintaining EMC within the railway infrastructure, though additional standards may also apply depending on the equipment's characteristics and operational environment.