GWR 3150 Class

The GWR 3150 Class was a series of large outside-cylinder 2-6-2 Prairie side-tank locomotives designed by George Jackson Churchward and introduced at Swindon Works in 1906, representing the larger of Churchward's two Prairie tank designs and built for the heavier outer-suburban and semi-fast passenger working on the GWR's main and secondary lines. The Prairie tank — 2-6-2T — gave six coupled wheels for adhesion on heavier trains, a leading pony truck for stable guidance at speed, and a trailing pony truck that allowed a deeper, wider firebox than the 2-6-0 Mogul tender engine layout could accommodate in a compact tank engine.

Churchward introduced both large Prairie (3150 Class) and small Prairie (4400/4500 Classes) types to cover the range of GWR suburban and branch passenger requirements. The 3150 Class was designed for the busier outer-suburban services in the London, Birmingham, and Bristol areas where longer-distance semi-fast working demanded more power than the smaller Prairie could provide. With 5 ft 8 in coupled wheels and outside cylinders fitted with Churchward's long-travel valve gear, the 3150 gave free-running performance at the moderate speeds of outer-suburban working while the side-tank configuration provided ample water capacity for the longer outer-suburban turns.

The 3150 Class gave long and reliable service on GWR and BR outer-suburban and secondary passenger duties. Further batches were built through the 1910s and early 1920s. Several survived into the late 1950s. None was preserved, though the closely related 5100 and 6100 Large Prairie classes of the Collett era are represented in the preserved fleet.