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 power class 1 with a maximum of 1,000 hp and was intended for lighter operation with mixed freight trains. Thus, it did not receive a train heating boiler 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. Due to the previously developed diesel locomotives in this power class not being a success, 128 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 as reinforcements in 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 head codes.
A further 100 were built between 1965 and 1968, bringing the total to 228. Some came with slow-speed control 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 operated with three locomotives.
At the end of the British Rail era, they were rarely found on main lines 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 line from London to the Eurotunnel. Some were even used temporarily in France for this task. Direct Rail Services operated nuclear flask 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, also the delivery services of the new S-stock cars for the London Underground were already history by this time. 22 are preserved for posterity. They are all operational, either only for individual heritage lines or with main line certification.