The Class 20, officially known as the English Electric Type 1, was the first mass-produced British mainline diesel locomotive. It was at the top end of performance class 1 with a maximum of 1,000 hp and was intended for lighter operation with mixed freight trains. Thus, a boiler for passenger trains was dispensed with and the power was drawn from the eight-cylinder version of the English Electric diesel engine. The engine code 8SVT indicated a supercharged V8 engine. Its distinct sound led to the nickname "Chopper".

Unusual for British locomotives, only one cab was installed at one end of the locomotive, with one driver's desk for each direction. Since the transition from steam to diesel was just beginning at the time, the poor visibility in one direction was manageable. Since the previously developed diesel locomotives in this power class were not a success, 128 units were ordered, which were delivered between 1957 and 1962.

As planned, the locomotives were primarily used for freight traffic and were usually running cab-first. In the summer they were also used to increase passenger traffic, and their duties also included shunting empty passenger trains. The lighting system initially consisted of the multitude of lamps to form combinations for identifying the type of train, as was common on British steam locomotives. From 1960 these were replaced by the large boxes for displaying the train numbers.

A further 100 examples were built between 1965 and 1968, bringing the total to 228. Some came with slow-speed controls for loading and unloading coal trains. When the light mixed freight trains became fewer and fewer due to the growing road traffic and light mainline locomotives were no longer needed, the locomotives were increasingly seen in pairs in front of heavier trains. They were coupled together with the cabs on the outside to allow sufficient visibility in both directions. Some of the coal trains in the Scottish lowlands were even pulled by three locomotives.

At the end of the British Rail era, they were rarely found on main routes in front of regular trains. After privatization, the Class 20 was increasingly used by its new operators for special tasks. Some were used on construction trains, helping to build the Eurotunnel and the High Speed 1 route from London to the Eurotunnel. Some were even used temporarily in France for this task. Direct Rail Services operates nuclear waste container trains with one locomotive at each end. The DRS “Railhead Treatment Trains” also had a locomotive at each end, which used high pressure to clear the tracks of leaves in autumn at high speed. Class 20 locomotives have not been used in this role since 2020. At that time, the number of locomotives in general had shrunk considerably, and the deployments for the delivery of the new S-stock cars for the London Underground were already history by this time. 22 pieces are preserved for posterity. They are all operational, either only for individual museum routes or with certification for all main routes.