Designed as a more powerful, updated version of Churchward's renowned ‘Star’ class of 1906, the ‘Castles’ were an immediate success, and achieved an enviable reputation for speed during the inter-war years on such trains as the one-time world's fastest, the Cheltenham Flyer.
A total of 171 ‘Castles’ were in service by 1950, which included 25 converted from 'Stars' and one from ‘The Great Bear’, Britain's first 4-6-2 (Pacific) wheel arrangement engine. Eventually displaced by dieselization, eight engines of this class survived into preservation. Most were named after castles in the Great Western's territory, 12 after second world war aeroplanes, and 24 carried miscellaneous names. In 1937 it was decided, some say demanded, by certain Earls that new small engines intended to carry their names should not do so, instead 21 ‘Castles’, between 5043 and 5063, were commandeered and re-named.
5051 ‘Drysllwyn Castle’ was built at Swindon in May 1936. She was renamed ‘Earl Bathurst’ in August 1937 and carried that name for the rest of her GWR and BR life. Both names are regularly used on the locomotive at Didcot. Stationed almost exclusively at Landore (Swansea) depot, for working to London and the Midlands, she was rarely seen elsewhere.
Withdrawn by BR as surplus to requirements, from Llanelly depot in 1963, she was sent for scrapping to Woodham Bros. of Barry, from where it was rescued by a Society member, and brought to Didcot in February 1970. Out shopped in 1980 she took part in the Rocket 150 celebrations at Rainhill in May of that year. She then ran many main line specials between (in the early years with Didcot's vintage train) until 1986 when the main line ticket ran out. The engine then ran at Didcot until the boiler certificate ran out in 1990.
Following a second overhaul she was returned to service, and enjoyed runs out on the mainline once more, and visits to other preserved railways as well as Didcot duties. In 2008 the latest boiler certificate expired and the locomotive is now on static display until such time as a further overhaul can take place.