Designed by Robert Urie for the LSWR as a general purpose locomotive the S15 followed the 2-cylinder 4-6-0 format which was to remain the most common type of locomotive built for secondary service until the end of steam. The design provided a good combination of power, free-running, free-steaming and ease of maintenance, culminating in the LMS 'Black 5' and its BR standard derivatives. These mixed traffic locomotives could run fast enough to haul heavy secondary and cross-country passenger services over long distances and speed up delivery of express goods trains while being powerful enough to haul most of the ;general goods trains on the Southern. On the formation of the Southern Railway in 1923 the S15 was selected as a standard type, further batches being ordered by SR CME Richard Maunsell, expanding the class to 45 examples from which 7 have suvivied into preservation.