The 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.
The May 1936 design was based on the 1924 proposal for a 2-6-0 with 5 ft 2 in diameter coupled wheels, but with K3 cylinders, a K2 boiler, and a B17 firebox. The frame was 5 inches longer than the K3, with a design boiler pressure of 180 psi giving a tractive effort of 32,939 lbf, and an estimated factor of adhesion of 3.92.
Gresley later raised the steam pressure to 200 psi which saw the tractive effort leap to 36,598 lbf, with a corresponding reduction in the factor of adhesion to 3.54.
6 were built, 1 has been preserved.
