Work on the dual power concept continued throughout the 1950s, during which time the straight electric class 71 was also designed and introduced. The class 71 attempted to solve the absence of third rail issue with additional catenary pick up but the overhead lines were only installed in a limited number of siding locations.

Finally an order for six electro-diesel locomotives was made in July 1959. These were initially designated class JA, subsequently under TOPS initially class 72 but 73/0 was the classification that was implemented. They were numbered E6001-6, subsequently 73001-6. These six were built at the SR's Eastleigh works and are capable of 1600hp (i.e. similar to the Southern's native diesel electric class 33) on straight electrical pick up and 600hp from its own diesel-electric generator set. In tune with the Southern's policy of standardisation and interchangeability of parts the diesel-electric generator set was from the same series as used in the DEMU units and the traction bogies (albeit with different pick up arrangements) as on contemporary EMU and DEMU stock. The class was designed to work in multiple with all other Southern MU stock - with other JAs, EMUs, DEMUs and subsequently JBs, 33/1s and 74s. A strong point of the design is that the locomotives can change their power at speed since the diesel can be started and stopped and the pick-ups raised and lowered on the move.

photograph by Ray Soper

photograph by Alan Robinson

photograph by Mike Morant

photograph by Michael Taylor