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Concept
4-8-4
Summary
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 initially named the Northern Pacific, but railfans and railroad employees have shortened the name since its introduction. It is most-commonly known as a Northern.
The 4-8-4 wheel arrangement was a 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 a four-wheel trailing truck, which allowed greater production of steam. The four-wheel leading truck gave stability at speed and the eight driving wheels gave greater adhesion. The 4-8-4 type evolved in the United States soon after the Lima Locomotive Works introduced the concept of "Lima Super Power" in 1925, making heavy 2-8-2 and 2-8-4 locomotives. The prototype was built by American Locomotive Company (ALCO) for the Northern Pacific Railway (NP) in 1926, with a very large firebox with a grate, designed to burn low quality lignite coal. The four-wheel trailing truck weighed about more than two-wheel trucks of the time and could carry an additional of engine weight; the difference of was available for increased boiler capacity.
The 4-8-4 type arrived when nearly all the important steam locomotive design improvements had already been proven, including Superheaters, mechanical stokers, outside valve gear and the Delta trailing truck. One-piece, cast steel bed-frames with integrally cast cylinders gave the strength and rigidity to use Roller bearings. In 1930, the Timken Company used the Timken 1111, a 4-8-4 built by ALCO with roller bearings on all axles, to demonstrate the value of their sealed roller bearings. The Timken 1111 was subsequently sold to the NP, where it became NP No. 2626, their sole Class A-1 locomotive.
Official source
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