George Jackson Churchward created for the Great Western Railway a family of standard classes of locomotive, based on a limited set of shared dimensions and components, and his principles were followed by his successors.
Most of these locomotives had two cylinders, placed outside the frames, and they are listed here, ranging in size from small-wheeled 2-6-2T tank locos to 2-8-0 express freight engines.
Standardisation on the GWR started with Daniel Gooch, its first Locomotive Superintendent[a], who enforced uniformity on third-party suppliers of parts by issuing lithographed copies of drawings and iron templates to check accurate fitting.
Joseph Armstrong (and his brother George) used similar techniques at Wolverhampton, and continued Gooch's policy when he replaced him at Swindon.
William Dean, Armstrong's successor, designed (among others) four classes of locomotive, 2-4-0 and 0-6-0 tender and tank engines, which shared many standard components.