The Leader was a class of experimental 0-6-0+0-6-0T steam locomotive, produced in the United Kingdom to the design of the innovative engineer Oliver Bulleid.

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.

In an effort to demonstrate the continued potential of steam, Bulleid pushed forward the boundaries of steam-power, hoping it could compete with diesel and electric locomotives in terms of labour-saving and ease of operation.

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.

[4] By developing the proposal further, Bulleid settled for a 0-6-0+0-6-0T design of bogie locomotive, which spread the weight more evenly over the rails and reduced the axle-loading.

[5] A series of initial ideas were presented to the Southern Railway management by Bulleid that incorporated double-ended running, giving the locomotive driver maximum visibility in either direction without a boiler or tender obscuring his view.

The boiler was offset to provide space for a communication corridor, allowing the driver to access both cabs without leaving the locomotive, an arrangement which led to later problems.

[8] The entire ensemble was placed on a common frame and thus often referred to as an 0-6-6-0T, even though the actual notation is 0-6-0+0-6-0T since both engine units pivoted as on a Garratt, Double Fairlie or Meyer locomotive.

[13] Each of the two bogies had three cylinders, with the driving wheels connected by chains enclosed in an oil-bath, based upon Bulleid's chain-driven valve gear on his Pacific locomotives.

[14] The boiler pressure was set at 280 psi (1,900 kPa) and each was fitted with four thermic siphons within the firebox, both to increase the rate of evaporation and improve water circulation.

[21] The arch was problematic because it led to a tendency for flames to enter the cab at high outputs, a situation made worse by the narrowing of the firebox area.

This was a result of another Bulleid labour-saving innovation, a sliding hatch controlled from the front cab, that enabled ash to be cleaned out via a chute onto the track when the locomotive was on the move.

[23] For ease of maintenance, the boiler, firebox and smokebox were encased in steel sheeting, which meant that the engine's shape resembled that of a modern diesel locomotive.

[1] Entry into the locomotive was by way of ladders leading up to sliding doors, although, due to the bogie design, the climb into the fireman's cab necessitated clambering over the oilbath casing.

[28] However, the results of the trials as reported to British Railways headquarters at Marylebone were "conspicuous by the absence of praise" for the strengths of the Leader, namely the boiler, braking system and total adhesion provided by the two bogies.

36001 was shown to have several flaws, including heavy coal and water consumption, mechanical unreliability, untenable working conditions for both fireman and driver, loss of steam through the cylinder rings and uneven weight distribution on the bogies.

After renewing the cylinder assembly, it was tested around Brighton and Eastleigh using an LNER Dynamometer car, where good running was experienced at high costs in fuel and effort on the part of the fireman.

[23] It was an enclosed space that was constantly hot and the single fireman's entrance door on the side of the locomotive was left open during travel to promote ventilation.

[19] Measurements in the fireman's cab showed temperatures could reach 120 °F (50 °C) earning the locomotive the nickname of The Chinese Laundry due to the heat and humidity.

[27] Experiments had to be undertaken to balance the locomotive by filling the linking corridor with large quantities of scrap metal, replaced in a re-design by a raised floor, covering the weighted material.

To circumvent this problem the locomotive was used in reverse, as the rear cab was next to the water tank and coal chute and therefore away from the hot gases circulating inside the smokebox.

[1] Bulleid advocated a continental style of locomotive nomenclature, based upon his experiences at the French branch of Westinghouse Electric before the First World War.

[6] If the class had gone into full production, the locomotives would have been painted in British Railways mixed traffic/freight black livery with red, grey and cream lining.

[3] The whole concept was quietly dropped in 1951 after Bulleid left British Railways to become Chief Mechanical Engineer of Córas Iompair Éireann (where he produced CIÉ No.

[40] R. G. Jarvis, who was placed in charge of the project after Bulleid's departure, insisted that the locomotive required an entire re-design to solve the problems of the original concept.