For the Southern Region, where the principle power source was electric, a dual-power (electric/diesel) loco was considered an advantage. The 3rd rail could provide plenty of power, while the diesel engine would power the locomotive where the 3rd rail couldn't reach in yards, branchlines and sidings. Plans for a dual power locomotive were first considered in the late 1930s, but it was not until the mid-1950s that a firm project was proposed. In July 1959 approval was given to construct six prototype dual-power locomotives, a 1,600bhp electric locomotive fitted with an an English Electric 4SRKT auxiliary engine, delivering 600bhp. E6001-E6006 were constructed at Eastleigh Works, the first completed February 1, 1962 painted in BR EMU green with small yellow warning panels. All six locomotives were in service by the end of 1962, based at Stewarts Lane.
To maximise the flexibility of the locomotives control equipement was installed to permit multiple working with BR Southern-type EMU stock in addition to the BR 'Blue Star' diesel locomotive control system, plus vacuum braking and control values to adapt between air and vacuum brake systems. BR(S) was so pleased with their 'ED' fleet that 43 near identical production locomotives were ordered 1964. E6007 to E6049 were built by English Electric at the Vulcan Foundry, Newton-le-Willows, the first locomotive being delivered in October 1965. The production batch was practically identical to the Eastligh built prototypes, noticable changes being slight window/grille alterations, removal of the multiple control jumper from below the driver's window and modified bogie design. A number of technical developments and modifications were also incorporated.
The fleet was classified 73 under the BR TOPS system with sub-classes 73/0 (73001-6) for BR built locomotives and 73/1 (73101-42) for the EE locomotives.
These locomotives have undertaken a wide variety of work in their long lives, covering mundane goods trains to the Orient Express, Britains best known luxury train.