The ten prototypes with the numbers D200 to D209 were delivered in 1958 as the first large diesel locomotives to be built in series by English Electric. They were based on the three experimental D16/2s completed in 1950 and 1954. The third example already had the 16SVT with 2,000 hp, which, like the bogies, was used for the production locomotives. The bogies, each with three powered axles, also had a carrying axle each in order to be able to carry the heavy weight. The locomotive body of the production locomotives deviated from the D16/2 and already had the shapes that were later to be found on the classes 37 and 55.
Another 190 production locomotives followed until 1962 as the D210 to D399, which later became known as the class 40. The first batch only received a vacuum brake system, the later ones could also pull cars with air brakes. Since they were intended for use in front of passenger trains, all received steam heating. Even during their testing, some people who considered the power for a large locomotive with such a high weight as too low. During the production of the last batch, the locomotives of classes 47 and 55 were introduced, which developed a significantly higher power at a lower weight without unpowered axles.
The Class 40 was primarily used in the north of England in front of high-value passenger trains. On the large main routes they were soon replaced by more powerful diesel locomotives, but on other routes they initially had no major competition due to their low axle loads. The lack of electric train heating became their problem from around 1980, as fewer and fewer passenger cars with steam heating were available. In contrast to the class 37, the class 40 was not modernized, so that larger numbers of decommissioning began and in 1985 the last examples disappeared from the scene. A total of eight locomotives were preserved, some of which are still operational today.