Under the terms of the Railways Act 1921, the Great Western Railway (GWR) amalgamated with six companies - the "constituent companies" - and absorbed a large number of others - the "subsidiary companies". All of the constituent companies and ten of the subsidiary companies owned locomotives, ranging from the Taff Vale Railway which had 275 locomotives (one of which was not taken into GWR stock), to the Cleobury Mortimer and Ditton Priors Light Railway and the Gwendraeth Valleys Railway, with just two each. The constituent companies were amalgamated on 1 January 1922, some of the subsidiary companies being absorbed on the same date, the rest following at intervals until July 1923. Two more undertakings, not mentioned in the Act, which were responsible for shunting at Swansea Docks, sold their locomotives to the GWR soon afterwards.
Allcock, N.J.; Davies, F.K.; le Fleming, H.M.; Maskelyne, J.N.; Reed, P.J.T.; Tabor, F.J. (June 1951). White, D.E., ed. The Locomotives of the Great Western Railway, part one: Preliminary Survey. Kenilworth: RCTS. ISBN0-901115-17-7.
Davies, F. K.; Firth, J. M.; Lucking, J. H.; Thomas, R. E.; Allcock, N. J.; Sterndale, A. C.; Barrie, D. S. M.; Reed, P. J. T.; Mountford, E. R. (April 1966). White, D. E., ed. The Locomotives of the Great Western Railway, part ten: Absorbed Engines, 1922–1947. RCTS. ISBN0-901115-20-7.