On taking up office in 1922, Collett began to develop the more powerful GWR Castle class from Churchward's Star class. However, the design was limited to a maximum axle-loading of due to the weakness of some underline bridges. The new class would not therefore be able to pull 13+ carriage express trains unaided. Following their introduction in 1923 the Castle Class was the most powerful express passenger class in the country in terms of tractive effort, but this honour was lost to the Southern Railway Lord Nelson class in 1926.