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The SR Leader class was a class of experimental 0-6-6-0 articulated steam locomotive, produced to the design of the innovative engineer Oliver Bulleid. Intended as a replacement for the ageing fleet of M7 class, the Leader was an attempt to extend the life of steam traction on the Southern Railway by eliminating many of the operational drawbacks associated with existing steam locomotives. Design work began in 1946, and development continued after the nationalisation of the railways in 1948, under the auspices of British Railways. The Leader project was part of Bulleid's desire to modernise the steam locomotive based on experience gained with the Southern Railway's fleet of electric stock. The design incorporated many novel features, such as the use of thermic siphons, bogies, and cabs at either end of the locomotive, resulting in its unique appearance. Several of its innovations proved to be unsuccessful however, partly accounting for the project's cancellation in the early 1950s. Five Leader locomotives were begun, although only one was completed. Problems with the design, indifferent reports on performance, and political pressure surrounding spiralling development costs, led to all locomotives of the class being scrapped by 1951.