The London and North Eastern Railway (LNER) Thompson B1 is a class of steam locomotive designed for medium mixed traffic work. Designed by Edward Thompson, it was the LNER's equivalent of the highly successful GWR Hall Class and the LMS Stanier Black Five, two-cylinder mixed traffic 4-6-0s. However, it had the additional requirement of having to be cheap because, due to wartime and post-war economies, the LNER was short of money.
The prototype for the new B class (later classified B1) 4-6-0 was built at Darlington and entered service on 12 December 1942. It was the first 2-cylinder main-line locomotive constructed for the LNER since the grouping, such had been Sir Nigel Gresley's faith in the 3 cylinder layout. Constraints on production meant that the first ten were not completed until 1944. However, Thompson then placed substantial orders with two outside builders: Vulcan Foundry and the North British Locomotive Company of Glasgow. Between April 1946 and April 1952 NBL built 290 B1s. Orders for the B1s, which became Nos. 61000–61409 under British Railways, totalled 410.
The B1s operated throughout LNER territory. The first batch was distributed among depots on the former Great Eastern Railway section: Ipswich, Norwich, and Stratford in London. They were an immediate success and were soon working the Liverpool Street - Harwich boat trains, the Hook Continental, the Day Continental and the Scandinavian. B1s were also a familiar sight on other top-link workings such as The East Anglian, The Broadsman and The Fenman. During the 1950s over 70 B1s were stationed on ex-GE lines. They enjoyed similar popularity on ex-Great Northern and Great Central territory. Sheffield based B1s were regularly rostered for the Master Cutler and South Yorkshireman expresses. Elsewhere there were substantial allocations in Scotland, West Yorkshire and East Yorkshire.
Overall, however, it was entirely necessary that the B1s be introduced, because the LNER was operating a large number of engines that were well past their economic life. Ironically, the B1s were also withdrawn long before their projected economic working life. Excepting 61057 which was destroyed in an accident in 1950, the first normal withdrawal was 61085 in November 1961. The remaining locomotives were withdrawn between 1962 and 1967. Two have been preserved, 61264 and 61306, both built by the North British Locomotive Company.