In 1941 the Southern Railway introduced its first goods locomotive for use on its third rail DC electrified system. Lots of short gaps in the third rail can be bridged by the length of multiple units with contact shoes at both ends, but a shorter locomotive would have faced a short loss of tractive power while running over each gap. While the kinetic energy of the train was usually enough to coast over the gap, this led to a jolt going through a loose coupled train, what created the risk of damage to the couplers or the transported goods.

The solution was the use of a flywheel that had an electric motor and a generator mounted on one shaft, two of which were mounted in the locomotive. Excess power from the third rail was used to spin the flywheels and the moment the locomotive ran over a gap in the power supply, the stored energy was used to power the generators and in turn the traction motors. In this way, a nearly steady tractive force could be sustained. Additionally, the rotating flywheels could be used as booster, increasing the voltage collected over the third rail from 660-750 to 1,200 V for a short time.

In yards, where a third rail would pose a hazard for the personnel, the locomotive drew its power via a roof-mounted pantograph from overhead wires. Six traction motors with 245 hp each produced a total power of 1,470 hp. Primarily intended for goods trains, it also had train heating boilers for use with passenger trains. Approved for 75 mph (121 km/h), it demonstrated good running characteristics at speeds of up to 90 mph (145 km/h). Although the locomotive was originally planned to have four axles, it had to get six axles due to the weight of its equipment.

Inspired by the French numbering scheme, it was designated CC1 due to its C-C wheel arrangement. CC2 followed in 1945 and a third one was delivered to British Railways in 1948. They now carried the numbers 20001 to 20003 and the last one had a revised exterior design. Additionally, there had been incremental improvements in the electric system of the three locomotives. Although all three were withdrawn between October 1968 and January 1969 before they got TOPS numbers, they are today being known was class 70. The class 71 also used flywheel boosters, but was not technically related to the class 70.