4-8-4

4-8-4 (Northern)

Santa Fe no. 3751 of 1927

Equivalent classifications UIC class 2D2 French class 242 Turkish class 48 Swiss class 4/8 Russian class 2-4-2

First known tender engine version First use 1926 Country United States of America Locomotive NP class A Railway Northern Pacific Railway Designer American Locomotive Company Builder American Locomotive Company Evolved from 4-8-2

Under the Whyte notation for the classification of steam locomotives, 4-8-4 represents the wheel arrangement of four leading wheels on two axles, eight powered and coupled driving wheels on four axles and four trailing wheels on two axles. The type was first used by the Northern Pacific Railway and is consequently most commonly known as a Northern.

The 4-8-4 wheel arrangement was an obvious progression from the 4-8-2 Mountain type and, like the 2-8-4 Berkshire and 4-6-4 Hudson types, an example of the "Super Power" concept in steam locomotive design that made use of the larger firebox that could be supported by the four-wheel trailing truck, which allowed improved free steaming, particularly at speed. This was combined with the stability at speed brought about by the four-wheel leading truck and the greater adhesion of the eight driving wheels. The evolution to the 4-8-4 type occurred in the United States of America soon after the Lima Locomotive Works introduced the concept of "Lima Super Power" in 1925. The prototype was built to Super Power principles by American Locomotive Company (ALCO) for the Northern Pacific Railway (NP) in 1926, with a four-wheel trailing truck to carry the weight of a very large firebox that was designed to burn low quality lignite coal. The potential benefit of supporting a firebox with a 100 square feet (9.3 square metres) grate on a four-wheel trailing truck was quickly realised by locomotive designers since, given the truck’s additional weight of approximately 15,000 pounds (6,804 kilograms) and the 55,000 pounds (24,948 kilograms) of additional engine weight that a four-wheel truck could carry above that of a two-wheel truck, the difference of 40,000 pounds (18,144 kilograms) was available to be used for increased boiler capacity.