The 33 British Rail Class 43 diesel-hydraulic 'Warship' locomotives were built by the North British Locomotive Company (NBL) from 1960–1962. They were numbered D833-D865 and named after British Navy warships. They appeared in the green, maroon and blue liveries associated with the Western Region during the 1960s. Diesel hydraulic locos have a fanatical following due to their quirky, non-standard nature within the BR diesel fleet.
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 originally numbered D800-D832 and D866-D870. They were allocated Class 42 under the 1968 classification system, while those built by the North British Locomotive Company (NBL) were originally numbered D833-D865 and allocated Class 43. Because of their early withdrawal dates, neither the Swindon- nor the NBL-built locomotives carried TOPS numbers. Figures for 1965 show the North British Warships covered a far greater annual mileage than contemporary Type 4's such as the Westerns, Peaks and Brush Type 4. Despite being their last year in service, Class 43 locomotives were still hauling long distance passenger trains over the summer of 1971 on services between Paddington and locations in Devon.