GWR 6100 Large Prairie Class

The GWR 6100 Class was a series of large outside-cylinder 2-6-2 Prairie side-tank locomotives designed by Charles Collett and built at Swindon Works from 1931, constructed for the intensive outer-suburban passenger services in the London suburban area from Paddington — the Reading and Oxford semi-fast trains, the Slough and Windsor local workings, and the longer-distance outer-suburban services on the GWR's Thames Valley lines that required a powerful and fast tank engine capable of sustained running at near-express speeds.

The 6100 Class was a development of Churchward's 3150 Class and the intermediate Collett 5100 Class, with a larger boiler and higher boiler pressure giving more steam for the intensive outer-suburban running pattern of the Paddington services. The outside-cylinder specification with Churchward-derived long-travel valve gear gave free-running performance at the 70 mph or more that the Thames Valley outer-suburban services could require, and the 5 ft 8 in driving wheels gave the right combination of tractive effort and speed for accelerating quickly from the frequent stops of suburban service while maintaining fast inter-station speeds.

The 6100 Class worked the GWR's London outer-suburban and Thames Valley services through the 1930s, 1940s, and 1950s, being the standard Paddington outer-suburban type until dieselisation replaced steam on the Western Region in the early 1960s. Several examples are preserved, making the 6100 Class one of the better-represented GWR suburban tank types in the heritage fleet.