In total, locomotives in the class were built by English Electric between and , the large number being in part because of the failure of other early designs in the same power range to provide reliable locomotives.
The locomotives were originally numbered DβD and DβD They are known by railway enthusiasts as "Choppers". Designed to work light mixed freight traffic, they have no train heating facilities.
Locomotives up to D were fitted with disc indicators in the style of the steam era; [ 6 ] when headcodes were introduced in the locomotive's design was changed to incorporate headcode boxes. Although older locomotives were not retro-fitted with headcode boxes, a few of the earlier batch acquired headcode boxes as a result of repairs. Unusually for British designs, the locomotive had a single cab.
This caused serious problems with visibility when travelling nose first, though in these circumstances the driver's view is comparable to that on the steam locomotives that the Class 20s replaced. It was common, however, to find Class 20s paired together at the nose, with their cabs at opposite ends, ensuring that the driver could quite clearly see the road ahead, and a guard can watch the train from the other locomotive without the need for a brakevan.
The Class 20 saw only limited service on passenger trains. A small number were fitted with a through pipe for steam heating , primarily for use in conjunction with a Class 27 locomotive on the West Highland Line. Otherwise their use was limited to summer relief services, particularly to Skegness often under the adopted title of The Jolly Fisherman starting from various places including Burton-on-Trent , Stoke-on-Trent , Derby and Leicester.
