The GWR 633 Class were 0-6-0Ts designed by George Armstrong and built at the Wolverhampton railway works of the Great Western Railway between November 1871 and April 1872. These were always Southern Division locomotives, but over the years some were fitted to work the Metropolitan lines and played a large role in the transportation of goods from Acton to Smithfield. Unusually for the GWR, they had side tanks instead of saddle tanks and inside frames. Their wheels were 4 ft 6+1⁄2 in (1,384 mm) in diameter and wheelbase was 15 ft 6 in (4.72 m), with a weight of 34 long tons 12 cwt (35.2 t; 38.8 short tons). There were twelve locomotives numbered 633–644.