The BR class 73s are Electro-diesels, meaning they are bi-mode locomotives which can run on diesel power or via the Southern Region's third rail electric supply. Some have been rebuilt with different engines and designated class 73/9
https://en.wikipedia.org/wiki/British_Rail_Class_73
The bulk of the active class 73 fleet is operated by GBRf, plus locomotives in use with Network Rail, Southern, South Western Railway & Transmart Trains (on hire at Eastleigh Works).
Tony