The United States Army Transportation Corps built 382 S100 Class 0-6-0 tank engine 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.^[2]

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 be able to haul heavy goods trains as well as full-length passenger trains in the harbour area.^{[1][page needed]} The railway’s Chief mechanical engineer, 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 0 in (3.35 m) wheelbase and inside cylinders, and also many of the survivors were in poor condition.^[2] However, the S100s stored at Newbury Racecourse had a 10 ft 0 in (3.05 m) wheelbase, outside cylinders and had hardly been used. Those available for sale had been built by the Vulcan Iron Works of Wilkes-Barre Pennsylvania and H. K. Porter, Inc, of Pittsburgh.

Bulleid therefore took Vulcan-built locomotive WD4236^[4] 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 thirteen were purchased in 1947 for £2500 each.^[4] Six of these had been built by Porter and the remainder by Vulcan. However, when it was discovered that there were differences in dimensions between the locomotives from different builders the SR exchanged its Porter built locomotives for Vulcans, but could only to do so with five examples. The railway therefore accepted one Porter locomotive at a reduced price and purchased another to provide spare parts.^[2] Thus, the thirteen further locomotives entered traffic between April and November 1947 as soon as they had been adapted.