The LSWR S15 class is a British 2-cylinder 4-6-0 freight steam locomotive designed by Robert W. Urie, based on his H15 class and N15 class locomotives. The class had a complex build history, spanning several years of construction from 1920 to 1936.

Maunsell's modifications included increasing the boiler pressure to 200 psi, and the reduction of the cylinder bore by half an inch. The footplate was also modified for operation on the Southern's new composite loading gauge. Other modifications included the lengthening of valve travel and fitting larger outside steam pipes to streamline the flow of steam into the cylinders.

Fifteen locomotives of this revised design were built in 1927, and some were given 4,000-imperial-gallon six-wheeled tenders for use on the Southern Railway's Central section. This allowed the locomotive to be turned on the shorter turntables found on this part of the network. From new, the rest of the class was equipped with the Urie 5,000-imperial-gallon eight-wheel bogie tender, which allowed the class to operate on the extended freight routes of the Southern Railway's Western section.

The standardisation measures undertaken by both Urie and Maunsell were soon vindicated by the fact that tenders and other parts were swapped with those of other classes on the Southern Railway when locomotives were under overhaul.

A third batch was ordered in 1931, coinciding with a downturn in the volume of freight due to the Great Depression. This meant that the last of the S15 class was not completed until 1936, although weight-saving modifications were undertaken to this batch. A final modification was also applied to the class at this time, when all locomotives were equipped with smoke deflectors to improve visibility from the footplate when travelling at speed. This modification was a feature that became common to most Maunsell-influenced designs.

Four have been preserved and an additional one, no. 841 has been cannibalised to keep no. 825 going.