In fact the 2021s were simple enlargements of the Armstrong-designed 850 class of 1874. The changes were fundamentally confined to a longer wheelbase to permit fitting of a larger firebox.
Rebuilding with Belpaire fireboxes commenced in the early years of the Churchward era. Unsuccessful attempts to form a saddle tank around the firebox directly led to the switch to pannier tanks. The first pannier tank conversions occurred in 1912, and rebuilding of the majority of the class took place over many years – the last conversion was in 1948, and some were still saddle tanks when withdrawn.[8] In their final form, with or without fully enclosed cabs, 110 of them survived into British Railways ownership, the last of them being retired in 1959.[9] They were superseded by the short-lived GWR 1600 Class, nominally a Hawksworth design, but in reality a straightforward update of the then 75-year-old design, with new boiler, bigger cab and bunker.
When autotrains were introduced on the GWR, a trial was made of enclosing the engine in coachwork to resemble the coaches. Nos 2120 and 2140 of this class were so equipped in 1906, as were two 517 class 0-4-2Ts. The experiment was unpopular with engine crews, and the bodywork removed in 1911.[10][11]
^Norris, John (1987). Edwardian enterprise : a review of Great Western Railway development in the first decade of this century. Didcot: Wild Swan. p. 127. ISBN0906867398.
Ian Allan ABC of British Railways Locomotives, 1948 edition, part 1, pp 16,51
le Fleming, H. M. (April 1958). White, D. E. (ed.). The Locomotives of the Great Western Railway, part five: Six-coupled Tank Engines. RCTS. ISBN0-901115-35-5. OCLC500544510.
Whitehurst, Brian (1973). Great Western Engines, Names, Numbers, Types and Classes (1940 to Preservation). Oxford, UK: Oxford Publishing Company. ISBN978-0-9028-8821-0. OCLC815661.
