The LMS Royal Scot class were built to haul the London to Scotland expresses on the LMS west coast route, but had been displaced by the new Stanier pacifics in the 1930s. The class had not been particularly outstanding in performance and Sir William Stanier began a rebuilding programme for the class, fitting a more modern tapered boiler with Belpaire firebox to improve steaming capacity, better suiting the Royal Scots to the secondary express services needed during the 1940s.