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.