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 . The British Rail Class 73 is a United Kingdom model of electro-diesel locomotive . The type is unusual in that it can operate from a 750 V DC third-rail or an on-board diesel engine to allow it to operate on non-electrified routes .