LNER Class K4

The London and North Eastern Railway (LNER) Class K4 is a class of 2-6-0 steam locomotives designed by Nigel Gresley for the steep grades of the West Highland Line.

West Highland Line challenge

The North British Railway West Highland line to Mallaig via Fort William, presented a combined triple operating challenge of: steep gradients; severe curves; and restrictive axle loading limits. Having used D34 'Glen' 4-4-0s, increased loads led to regular double-heading. Locomotive engineers proposed use of LNER Class K3, but they would not have been permitted to operate between Fort William and Mallaig.

Having proposed a new design based on a K3 boiler, in October 1924 a loan was made of a single LNER Class K2, which provided the required increase in power and adhesion. As K3s replaced K2 on the network, more K2s were loaned to the line, with the loan becoming permanent from October 1925.

With further increases in load and needs for additional traffic speed, in September 1934 Gresley instructed Doncaster Works to investigate the possibility of increasing the tractive effort of the K2s. After recommending against a design which increased boiler pressure and cylinder diameter to , in 1935 the Joint Traffic & Locomotive Committee signed off provision of a new design by reducing the 1936 build of K3s from 21 to 20.

The eventual May 1936 design was based on the 1924 proposal for a 2-6-0 with diameter coupled wheels, but with K3 cylinders, a K2 boiler, and a B17 firebox. The frame...
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