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 (NBR) 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 was longer than the K3, with a design boiler pressure of giving a tractive effort of , and an estimated factor of adhesion of 3.92. Prototype The prototype K4 No.3441 left Darlington for Eastfield depot, Glasgow on 28 January 1937. After five weeks of crew training and being confined to goods work it made its début on a passenger train on 4 March. It soon became apparent that the boiler pressure brought little improvement in average speeds over the existing K2, and that No.3441 responded sluggishly when up against the gradients of the West Highland line. Gresley reacted by raising the steam pressure to which saw the tractive effort leap to , with a corresponding reduction in the factor of adhesion to 3.54. The K4 could now demonstrate its true capabilities handling 300 ton trains and with maximum speeds around on level ground. An advantage of the newcomer was that