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.

GWR 3901 Class
Type and origin Power type Steam Designer William Dean, rebuilt by George Jackson Churchward Builder Swindon Works Order number Lot 167 Serial number not issued[2] Build date 1907 Total produced 20
Specifications Configuration: ​  • Whyte 2-6-2T Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge Leading dia. 3 ft 2 in (0.965 m) Driver dia. 5 ft 2 in (1.575 m) Trailing dia. 3 ft 2 in (0.965 m) Fuel type Coal Water cap. 1,500 imp gal (6,800 L; 1,800 US gal) Boiler Standard No 5 Boiler pressure 200 psi (1.4 MPa) Cylinders Two, inside Cylinder size 17 in × 24 in (432 mm × 610 mm)
Performance figures Tractive effort 20,155 lbf (89.65 kN)
Career Operators Great Western Railway Class 3900 Power class GWR: B Axle load class GWR: Blue Withdrawn 1931–1934 Disposition Scrapped

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.^{[3][4][5][6][7][8]}

Most remained in the Birmingham area until 1923, when they began to be sent elsewhere, and replaced by the Large Prairies. All the class was withdrawn between 1931 and 1934, and scrapped.^[8]