On opening, the Metropolitan Railway's trains were provided by the Great Western Railway, but these were withdrawn in August 1863. After a period hiring trains from the Great Northern Railway, the Metropolitan Railway introduced its own, Fowler designed, 4-4-0 tank engines in 1864. The design, known as the A class and, with minor updates, the B class, was so successful that the Metropolitan and District Railways eventually had 120 of the engines in use and they remained in operation until electrification of the lines in the 1900s.