The third of Robert Urie’s 4-6-0 designs for the LSWR and similar in appearance to the N15 class, the S15 shared many of its two predecessor’s components, such as in the boiler, cylinder, valve gear and tender designs. The only major difference in design for the S15 came in the smaller diameter of the driving wheels, this gave better traction, which was essential for the S15’s anticipated heavy freight workings to the south coast ports.
By May 1921, sixteen engines had entered service, mostly in the London area, but the 1923 Grouping of Railway Companies coincided with Urie’s retirement and further orders of the S15 were delayed as the new CME, Richard Maunsell, sought to improve the design, establishing trials to analyse water/coal consumption of the engines, along with their ability to run to a precise schedule.
When these trials showed that the S15 was the best available all-round freight locomotive, Maunsell ordered a further fifteen locomotives from the workshops at Eastleigh, the finished locomotives emerging throughout 1927 and 1928. Maunsell’s modifications to the S15 included increasing the boiler pressure and the reduction of the cylinder bore.