 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.The basis of the Leader originated from a 1944 review of the Southern Railway's steam locomotive fleet, resulting in a Southern Railway design brief which called for a high-powered locomotive requiring little maintenance to replace the ageing fleet of M7 class tank engines.The brief also stipulated that the locomotive would be used on both passenger and freight trains, requiring high route availability.