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 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.

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.

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.

Crews began to prefer the LNER Class V4, but their lower power restricted their use and the K4s retained their pre-eminence on the West Highland line until the 1947 arrival of the first B1 4-6-0s, which replaced the K4s from Glasgow to Fort William.

[1] 61994 is now retired and on public display in the Museum of Scottish Railways, Bo'ness, where it is receiving cosmetic restoration by a small team of dedicated SRPS volunteers.

The eventual plan will be for the owner, The Cameron Railway Trust, to house the locomotive in a museum to be built on John's Balbuthie farm, along with 60009 "Union of South Africa".