The LSWR Class S11 was a class of 10 4-4-0 steam locomotives designed for express passenger work by Dugald Drummond. They were introduced to services on the London and South Western Railway in 1903. None of the class survived into preservation after their brief career in British Railways ownership.

The problem of uniting both power and traction in a compact express passenger locomotive design had taxed the Locomotive Superintendents of the LSWR for many years.^{[2][page needed]} Joseph Beattie was the first to establish the LSWR's policy of using smaller wheeled locomotives to handle these steep gradients.^{[2][page needed]}

Dugald Drummond attempted to grasp the nettle by utilising his new T9 class over the arduous route. It soon became clear that despite the merits of the T9s for fast running on the various express passenger services to the west of England, the large wheels of the class were not suited for the task in hand. A new design of locomotive was needed that incorporated the desired improvements to enable fast running on gradients.^{[1][page needed]}

Drummond took the decision to construct a new class of ten 4-4-0s especially for this part of the LSWR network. The class was to incorporate the same frames as the T9, though smaller 6 feet 1 inch (1.854 m) driving wheels were substituted, whilst they also had balanced crank axles.^{[3][page needed]} The boiler was also of 5 feet (1.52 m) diameter, another feature that was different from the T9s, capped off with a dome and stovepipe chimney. Production began at Nine Elms in 1903, and had ended by the end of the year with a total of ten locomotives.^{[2][page needed]} All locomotives were fitted with the Drummond "watercart" eight-wheel tender for longer running on the LSWR network.

The locomotive was fitted with cross-water tubes fitted into the firebox, as featured on the T9 class, although feedwater tubes were not fitted.^[4] This was an attempt to increase the heat surface area of the water, which was achieved, though at a cost in boiler complexity. All ten were superheated between 1920 and 1922 by Robert Urie after took over from Drummond.^[5]

The S11s were regarded as good locomotives on expresses around the ports served by the LSWR.^{[2][page needed]} The class was at first rostered to the West Country in order to ply their trade upon the gradients they were designed for. However, it was soon realised that the smaller wheels and larger boilers were not living up to expectations of successful operation on the route. The class was slower and more cumbersome than their siblings. This led to crews displaying their preference for the T9's higher speeds downhill and on the level stretches of railway.^{[7][page needed]}

Another aspect that did not endear the class to crewmen on this route was the fact that with a larger boiler, the S11s were heavier on water, which was a vital consideration on a railway with no water troughs to replenish the supply, and therefore skilled use of the injectors was required.^{[3][page needed]} The boiler was mounted higher above the frames, which led to instability concerning the ride of the locomotives at high speeds.^{[1][page needed]} As a result, more care had to be taken by crewmen in approaching junctions and speed restrictions. Due to their early withdrawal period, none were preserved for posterity.