Northern Pacific class A

The locomotive class that gave the 4-8-4 “Northern” its name was the class A of the Northern Pacific. Introduced in 1926, they were the first locomotives of this wheel arrangement in the USA. ALCO built a total of only twelve in this and the following year. They originated from the 4-8-2 “Mountain” by enlarging the firebox and switching the single trailing axle for a two-axle truck. With this firebox, they were optimized for burning low-grade coal from the Rosebud mine in Montana. The truck had outside frames and contained a booster. 

With a firebox area of nearly 500 square feet and a grate of 115 square feet, these locomotives could increase the speed of heavy passenger trains on the mountainous lines of the Northern Pacific. Due to their limited cutoff, the actual starting tractive effort was lower than the automatically calculated value in the spec sheet. Their successors differed since the next 4-8-4, ex Timken 1111 acquired in 1930, was heavier and had roller bearings on its axles.