Steam_Mountain_484 MK2 3D-modell

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.