At the Grouping of Britain's railways in 1923 the Great Western Railway had an advantage over the other Big Four railway companies in that it retained essentially the same management structure that it had had pre-Grouping. Yet the next 25 years were arguably characterised by a series of mistakes in its engine building policy as the advances made by the other companies were largely ignored. This controversial new history of the GWR examines the role and policy of the two Chief Mechanical Engineers at the GWR in this period - Charles Collett (1922-41) and Frederick Hawksworth (1941-7) - following the legacy of the often pioneering work of the previous CME, George Churchward (1902-22). Collett had spent his entire working life at the GWR and his autocratic and introverted nature did not encourage innovation or outside influence. With little experience of the operational demands on the fleet, the long-term strategy of the engine building policy was neglected, as were the opportunities for efficiencies through standardisation. Stanier was a talented engineer at the GWR but moved to the LMS in 1932 and when Hawksworth took over from Collett, having worked many years under his shadow, the narrowness of his experience was a severe handicap. The constraints of the war gave him little opportunity to reinvigorate the company and improve its operating efficiency to match that of its rivals as his power and influence waned and he achieved little before the company was subsumed into the nationalised railway in 1948. This enlightening history brings a whole new focus to GWR history and will be fascinating reading for all enthusiasts of the railway and for more general readers.