By the late 1930s the Southern Railway's electrified system had expanded beyond the suburbs sufficiently to consider using electrical traction on more than just multiple units. Accordingly two experimental Co-Co mixed traffic electric locomotives were designed and constructed, these being a co-operation between Oliver Bulleid (Chief Mechanical Engineer - responsible for external design, mechanical parts and maintenance) and Alfred Raworth (Chief Electrical Engineer - responsible for the electrical systems). However a fundamental problem with third rail pick up locomotives had first to be solved. Since the distance between the outer pickups of any such loco - even long ones as these turned out to be - is much shorter than on multiple units the traction supply could be lost when the gap between conductor rails is longer than the distance between the locomotive's pickups . This situation is commonly referred to as "gapping". This was solved by instead of the traction supply being used, via control systems, to power the traction motors directly it was instead applied to drive two motor generator sets with heavy flywheels. Thus power to the traction motors would be maintained by the M-G sets being driven by the flywheel when the traction supply was briefly lost. These motor-generator-flywheel sets were referred to as "boosters" which also became the nickname for the class. These locomotives were also fitted with a pantograph for overhead pickup in sidings and depots where a conductor rail presented danger to staff.

These locomotives were very solidly constructed with solebars and main frames welded up from channel sections carrying a boxy body made of metal sheets fitted over a frame. They were 56ft 9in long, weighed 99 tons and had a tractive effort of 40000lb. There were cabs at both ends and their design was similar to contemporary EMUs (such as the early "Sheba" Subs) with a domed roof. Between the cabs there was the electrical compartment which also included an electrically heated boiler for carriage heating, however this was partitioned off to prevent water and steam ingress to the electrical equipment. The electrical systems and ancillary units were supplied by the English Electric Company and sub contractors. The plate frame bogies were of riveted and welded construction with five sandboxes per side and had BFB wheels of 3ft 6in diameter.

The first locomotive numbered CC1 (renumbered to 20001 by BR) emerged from Ashford works in 1941. Until 1942 it ran in photographic grey livery with three horizontal lining stripes, two on the body sides, one on the lower part of the roof, which were extended round the cab front, rounded down and brought to a point which in later years would be known as "speed whiskers". This was replaced by malachite green livery with "Sunshine" Southern lettering and yellow lines at solebar and cantrail level. The second locomotive CC2 (20002) emerged from Ashford in 1945 in malachite green livery. These two were used on express passenger and goods trains and with the benefit of experience were modified to improve their efficiency.

20002 at Three Bridges in the days before the headcode panel was fitted, and well before the advent of yellow ends!

photograph by Keith Harwood

Then in 1948 British Railways constructed a third locomotive, 20003. It looked significantly different with its cab front being similar to the latest EMUs, by this time being the slab front design with larger windows of the "Queen Mary" Subs. This locomotive was also 1ft 6in longer, weighed 105 tons, was capable of 45000lb tractive effort, had fewer sanding boxes and a larger boiler water tank on the underframe between the bogies. Included from scratch were refinements learned from the earlier two and more modern electrical equipment. 20003 was also introduced in malachite livery with yellow lining but with yellow British Railways lettering and numbering in the Gill Sans font.

Various modifications happened to these locomotives throughout their lives. MU jumpers were an early addition to CC1/2 as were marker lights. Three sandboxes from each bogie were also removed from the earlier pair. CC1 was built with a headcode panel which was subsequently modified to be a boxed headlight but later converted back to a headcode panel. CC2 (20002) received a headcode panel in later life but 20003 never received did They were classified as class 70 under TOPS although photographic evidence suggests they never carried a TOPS format number.

They settled down to a productive but unremarkable life doing exactly what they were designed to do. They were closely associated with Victoria-Newhaven boat trains, assuming this duty from the Brighton Atlantics in 1949. They were popular, powerful, reliable and efficient locomotives though no more were produced and the Southern's next electrical locomotives were the HA (later class 71) design introduced in 1959. CC1 and 2 were withdrawn by 1969 and curiously the younger CC3 in October 1968, despite their virtues all being victims (like other classes) of being non standard.

photograph by Keith Harwood

photograph by Mike Morant

From 1949/50 they adopted the initial black and aluminium colour scheme chosen by British Railways for diesel, electric and gas turbine locomotives, however prior to this in 1948/9 20002 carried an experimental light blue livery and was exhibited in this colour to the railway executive at Kensington Addison Road station. From the late 1950s they carried green livery (thought to be a modified malachite) with a red and white line half way up the side stopping short of the cab doors and a pale green frame. 20001 was withdrawn in BR blue with full yellow ends, by which time it had also gained twin air horns on the roof.