To get a new powerful goods locomotive in the context of his standardization efforts for the LSWR, Robert Urie designed the class S15 on the basis of the H15 an N15. It used many parts already used in at least one of these two classes, especially the boiler, cylinders and valve gear. The major change were the driving wheels, which now only measured five feet and seven inches (1,702 mm).

Now they had enough pulling power for heavy goods trains, but were still fast enough for express goods and the occasional passenger train. They gained the nickname “Goods Arthurs” due to their relationship to the class N15 “King Arthurs”. Before the grouping to the Southern Railway and Urie's retirement, 20 had been built at Eastleigh.

His successor Maunsell in the SR halted further manufacturing until he had made some changes to the design. These consisted of revised steam paths, a higher boiler pressure and some measures to decrease the loading gauge what included somewhat smaller cylinders. As a result, the new locomotives were more powerful despite the smaller cylinders. Some were given a shorter three-axle tender to enable them to be used in areas with smaller turntables.

All 45 locomotives came to British Railways in 1948. Withdrawals only started in 1962 and the last ones were gone by 1966. After that, a total of seven have been preserved, of which two were built in LSWR times and five in SR times. Only 30506 and 30825 are currently operational, but others are now either being overhauled or restored.