73122 at Bournemouth in October 1987. ©Phil Richards
The Class 73s are highly unusual electro-diesel locomotives built between 1962 and 1967, that can operate either from an on-board 600hp diesel engine or from 650/ 750V DC third-rail. Until the introduction of the Class 88, the 73s were unique in their electro-diesel functionality following the withdrawal of the Class 74. They produce significantly more power when on third-rail, so rarely leave the Southern regions - however, some are currently used on the Caledonian Sleeper service in Scotland. Later in life, some were converted to the 73/9 subclass which provided them with the AAR system for multiple working as well as new engines. Many are still in service to this day, whilst eight have been preserved.
(Information provided via Wikipedia)