The SR USA class are some ex-United States Army Transportation Corps S100 Class steam locomotives purchased and adapted by the Southern Railway (SR) after the end of the Second World War to replace the LSWR B4 class then working in Southampton Docks. SR staff nicknamed them "Yank Tanks".

The United States Army Transportation Corps built 382 S100 Class 0-6-0 tank engines for use in the Second World War. They were shipped to the British War Department in 1943 and stored awaiting the invasion of Mainland Europe. Most went overseas but some remained in store.

By 1946 the SR needed either to renew or replace the ageing B4, D1 and E1 class tanks used in Southampton Docks, but Eastleigh Works was not in a position to do so in a timely manner or at an economic price. The replacement locomotives would need to have a short wheelbase to negotiate the tight curves found in the dockyard, but also be able to haul heavy goods trains as well as full-length passenger trains in the harbour area. Oliver Bulleid therefore inspected the surplus War Department tank locomotives. The Hunslet Austerity 0-6-0ST locomotives stored at the Longmoor Military Railway proved to be unsuitable for dock work because of their 11 ft wheelbase and inside cylinders, and also many of the survivors were in poor condition. However, the S100s stored at Newbury Racecourse had a 10 ft wheelbase, outside cylinders and had hardly been used.

Bulleid therefore took Vulcan-built locomotive WD4236 on approval in May 1946 and tested it thoroughly over the next few months. When it was found to be suitable, this locomotive and a further 13 were purchased in 1947 for £2500 each and purchased another to provide spare parts. Thus, the thirteen further locomotives entered traffic between April and November 1947 as soon as they had been adapted.

Two JŽ class 62 locomotives built by the former Yugoslav Railways to an almost identical design have been acquired for use at the Shillingstone Railway Project and given British liveries. There are minor technical differences between these and the USA tanks. 30075 (formerly 62-669 built 1960- Undergoing overhaul) & 30076 (formerly 62-521 built 1954- Stored)