Two distinct types of locomotive have been allocated Class 43.

North British Built 'Warship Class'

The D800 series diesel-hydraulic 'Warship Class', of B-B wheel arrangement, was constructed by two different builders. Those locomotives built by British Railways at Swindon Works were allocated Class 42, while those built by the North British Locomotive Company (NBL) were allocated Class 43.

Like most NBL designs, these locomotives were not very successful, and were withdrawn before their Class 42 sisters, themselves doomed to a short life because of the decision to standardise on diesel-electric transmission for mainline locomotives. None have survived in preservation.

High Speed Train Power Car

When originally built, the production High Speed Train (HST) units were considered to be Diesel Electric Multiple Units, and were allocated Classes 253 and 254 for Western and Eastern services respectively. The power cars (of Bo-Bo wheel arrangement) were numbered 43002-43198 in the carriage number series.

In the 1980s, following problems with the power cars, unit formations were abandoned and from then on HSTs were considered to be formed of two locomotives with a rake of semi-permanently formed carriages in between. The power cars were not renumbered, but merely considered henceforth to be Class 43 locomotives.

The class were until recently the fastest diesels in the world with an absolute maximum of 148mph and 125mph regular service speed. In 2004, 196 class 43 locomotives are operating and are run by three private companies (Midland Mainline, Great North Eastern Railway and First Great Western).