Although it looked like a diesel locomotive, the Leader was a prototype for a new type of steam locomotive. With the advent of diesel and electric traction, Oliver Bulleid tried to increase the life of steam traction by reducing the workload of the crews. The idea was to place a steam locomotive on two three-axle bogies and hide both the boiler and fuel and water supplies under a common body. In this case the locomotive was designed to replace the M7 0-4-4T suburban tank and had also to be suited for goods service.

Each bogie was powered by three cylinders with sleeve valves. The axles were not coupled by rods, but by chains. This created running characteristics which were similar to electric and diesel-electric locomotives. Steam was provided by a compact boiler that had a working pressure of 280 psi and four thermic syphons. The firebox was in the middle of the locomotive and the working place of the fireman was directly behind it. The driver had one cab at each end to improve route visibility and to eliminate the need for turntables.

Construction of five locomotives was started at Brighton in 1946. Due to the founding of British Railways and shifted priorities, only one was completed in 1949 and numbered 36001, while work on all others was stopped. Although designed to weigh 120 tons for a good route availability, it actually weighed 150 tons, giving an axle load of 25 tons. Additionally, the valve gear turned out to be too complicated and unreliable and the locomotive was too high for many water cranes. It was tested until 1951 without ever being used in commercial service and then scrapped together with the uncompleted locomotives.