The Great Western Railway (GWR) 3901 Class is a class of 2-6-2T steam locomotives rebuilt from class 2301 'Dean Goods' 0-6-0 tender locomotives. In 1907, a surplus of Dean Goods locomotives, and a requirement for more suburban tank locos, led to the rebuilding of twenty of the Dean Goods into 2-6-2T 'Prairie' tank locos. The inside cylinders and motion were retained, the frames were extended at each end, and leading and trailing pony trucks added, as were a Standard number 5 boiler, full-length side tanks (with a large cut-out to give access for oiling the motion) and a bunker. Locomotives 2491-2510 were rebuilt and renumbered 3901–3920. The locomotives were allocated to the Birmingham area to replace older 2-4-2T locomotives, having the advantage of greater adhesive weight.