The British Rail Class 73 electro-diesel locomotives are very unusual in that they can operate from a 750 V DC third-rail supply but also have a diesel engine to allow them to operate on non-electrified routes. This makes them very versatile, although the diesel engine produces less power than is available from the third-rail supply so they rarely stray from the former Southern Region of British Rail. Following the withdrawal of the more powerful Class 74 electro-diesels in 1977, the Class 73 is now unique on the British rail system.