Collett's predecessor, George Jackson Churchward, had delivered to the GWR from Swindona series of class-leading and innovative locomotives, and arguably by the early 1920s the Great Western‘s 2-cylinder and 4-cylinder 4-6-0 designs were substantially superior to the locomotives of the other railway groupings. In 1922 Churchward retired, and Collett inherited a legacy of excellent standardised designs. But, with costs rising and revenues falling, there was a need to rationalise the number of pre-grouping designs and to develop more powerful locomotives. Collett was a practical development engineer and he took Churchward's designs and developed them – notably the Hall from the Saint class, and the Castle from the Star. He was also responsible for more humble locomotives, such as many of the pannier tank classes. By 1926 something bigger than the Castle class was required to haul heavy expresses at an average speed of. General ManagerSir Felix Pole told Collett to proceed with the design and construction of a "Super-Castle". The result was the King class 4-6-0 design which emerged from Swindon Works in June 1927, designed to the maximum loading gauge allowed by the GWR's broad gauge legacy. The locomotive had dimensions never previously seen, and represented the ultimate development of Churchward's four-cylinder concept. It was the heaviest 4-6-0 locomotive ever to run inthe United Kingdom, and had the highest tractive effort. Because of its scale, though, the King class was restricted to a limited number of the major GWR routes. In 1931, using the same maximum loading-gauge theory, Collett designed the GWR Super Saloons for use on the boat train services from London to Plymouth.
Legacy
A gifted technical engineer, who could look at existing designs and reliably improve them, Collett produced a standardized fleet of locomotives ideally suited to the GWR's requirements. He was able to extract substantial performance gains out of the Churchward designs, and the Castle class is noted for producing more power per lb of water than any locomotive built by any of the other members of the Big Four. Collett has received criticism by contemporary engineers and later railway historians for undertaking very little innovation in his designs, instead sticking with Churchward's style in every case. Arguably this meant that by the time Collett retired the superiority of Great Western locomotives was lost to more modern designs, particularly those of William Stanier, who worked at Swindon before moving to the LMS in 1932. He took Churchward's style with him, but developed it in line with continuing developments in steam locomotive technology.
