The SR Class W were 3-cylinder 2-6-4T tank engines designed in 1929 by Richard Maunsell for use on the Southern Railway. They were introduced in 1932 and constructed at Eastleigh and Ashford. The class was intended for short distance, inter-company/regional freight traffic transfer in London, and were standardised with parts from the N, N1, U and U1 classes. Background From the 1920s London was surrounded by a number of large freight Marshalling yards, where long-distance freight trains were brought and re-organised into new trains for onward transmission to their destinations. Three of these yards, at Feltham, Norwood and Hither Green, were operated by the Southern Railway. The requirement for a fast freight transfer locomotive between these yards, and across London to other railway yards, provided a challenge for Richard Maunsell. The electrified suburban lines were busy with commuter traffic which took priority over freight workings, and the London network abounded with junctions and signal stops. The design brief called for a powerful locomotive with good acceleration and adhesion: one that was able to climb the steep gradients over the flying junctions that were to be found all over the Southern Railway's London network. The ability to operate over tight curves and be able to pull away from a standstill on the aforementioned gradients were also key considerations that needed to be incorporated into the design. A tank locomotive design was also preferable since the journeys were relatively short, and the work would involve frequent changes in direction. Design The SR Z class was considered at first, but due to the specialist characteristics of the design as a yard engine for marshalling freight, they were not deemed fit for the purpose of hauling loads under the tight timings of London's railway system. The solution was a smaller wheeled version of the ill-fated K1 class with three cylinders to allow for better acceleration, with three sets of Walschaerts valve gear. The resultant W class was designed in 1929. As a result of the rebuilding of the 'River' K1 class 2-6-4 tank engines following the Sevenoaks railway accident, surplus bogies and leading wheels were available for the new design. A Cartazzi axle was used for the leading pair of wheels rather than a swivelling pony truck. The 2-6-4 wheel arrangement was settled upon as being ideal for intensive suburban routes, and the use of parts from the K Class and the same boiler