Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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. This makes it very versatile, although the diesel engine produces less power than is available from the third-rail supply so the locomotives 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 railway network. These locomotives were ordered as part of British Railways' 1955 Modernisation plan which included the extension of the Southern Region electrification to various main lines. They were intended as mixed-traffic locomotives, hauling parcels, freight and also passenger trains, usually on routes that included some non-electrified sections, such as boat trains.