LNER Class K4

LNER Class K4

The Great Marquess after working a railtour to Blaenau Ffestiniog, Wales

Type and origin Power type Steam Designer Nigel Gresley Build date 1937-1939 Total produced 6

Specifications Configuration 2-6-0 Gauge 4 ft 8 1⁄2 in (1,435 mm) standard gauge Driver dia. 5 ft 2 in (1.575 m) Loco weight 68.4 long tons (69.5 t; 76.6 short tons) Tender weight 44.2 long tons (44.9 t; 49.5 short tons) Fuel type Coal Boiler pressure 200 psi (1.38 MPa) Cylinders Three Cylinder size 18.5 in × 26 in (470 mm × 660 mm)

Performance figures Tractive effort 36,600 lbf (162.80 kN)

Career Power class 5P6F Disposition One preserved, remainder scrapped

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.

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 220 psi (1.52 MPa) and cylinder diameter to 21 inches (533 mm), 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 5 ft 2 in (1.575 m) diameter coupled wheels, but with K3 cylinders, a K2 boiler, and a B17 firebox. The frame was 5 inches (127 mm) longer than the K3, with a design boiler pressure of 180 psi (1.24 MPa) giving a tractive effort of 32,939 lbf (146.5 kN), and an estimated factor of adhesion of 3.92.

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 180 psi (1.24 MPa) 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 200 psi (1.38 MPa) which saw the tractive effort leap to 36,598 lbf (162.8 kN), 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 60 mph (97 km/h) on level ground. An advantage of the newcomer was that it used only marginally more coal in working 300 ton trains than the K2s did with considerably lighter loads.