The class 73 is a dual-power locomotive developed to bridge the non-electrified sections of the third-rail electrified network of the Southern Region. The main reason for their introduction was that large goods yards were not electrified. So the class 73 was mainly used under 630 to 750 Volts DC using retractable contact shoes and could start its diesel engine when needed.

In electric mode, it had an output of 1,600 hp. The diesel engine was a four-cylinder English Electric 4SRKT Mk II with 600 hp. This power was enough to move goods trains around yards at low speeds, but not to run trains on non-electrified mainlines. Even the 1,600 hp available in electric mode was only enough for shorter express trains, since the timetables in the Southern Region were made for four- or five-car EMUs with around 1,000 hp. 

The first six prototypes built in 1962 at Eastleigh were called class 73/0 and had a top speed of 80 mph. 43 more were built between 1965 and 1967 by English Electric at Vulcan Foundry which were called class 73/1 and had a top speed of 90 mph. They are equipped for multiple working with different types of multiple units and locomotives of the Southern Region.

Until 2005, they operated the Gatwick Express with the help of a class 489 Gatwick Luggage Van (GLV) that was a 500-hp driving trailer rebuilt from a class 414 multiple unit. GB Railfreight uses the class 73 mainly for engineering trains. In 2013, three were rebuilt to class 73/9 by replacing the 600 hp diesel with a 1,600 hp V8 built by MTU. Also designated class 73/9 are two rebuilt by Network Rail with two 750 hp Cummins QSK19 each. Several locomotives are preserved, although most preserved railways are not electrified and can only use the class 73 in diesel mode.