The first to be built, in August 1923, was number 4073 Caerphilly Castle; the last, in August 1950, was number 7037 Swindon. The class was thus built over a time span of over 25 years.
The Castle class was noted for superb performance on The Cheltenham Flyer during the 1930s: on June 6th 1932, pulled by 5006 Treganna Castle, the train covered 77 miles from Swindon to Paddington at an average speed of 81.68 miles per hour. This world record for steam traction was widely regarded as an astonishing feat.
A few survive in preservation as either working examples or static exhibits, including Caerphilly Castle, Clun Castle, Pendennis Castle, Nunney Castle, and Defiant.
In 1926, number 5000 Launceston Castle was loaned to the London, Midland and Scottish Railway where it ran trials between London to Carlisle. It was rumoured that LMS tried to persuade GWR to build a batch of Castles for use on the West Coast Main Line.
The design of the later Royal Scots were, to a large extent, based on the Castle class.
Withdrawal started in the 1950s; the last to be withdrawn was Clun Castle.
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