4-6-4
4-6-4 (Baltic, Hudson)
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Under the Whyte notation for the classification of steam locomotives, 4-6-4 represents the wheel arrangement of four leading wheels, six powered and coupled driving wheels and four trailing wheels. In France where the type was first used, it is known as the Baltic while it became known as the Hudson in most of North America.
The 4-6-4 tender locomotive was first introduced in 1911 and throughout the 1920s to 1940s, the wheel arrangement was widely used in North America and to a lesser extent in the rest of the world. The type combined the basic design principles of the 4-6-2 Pacific type with an improved boiler and larger firebox that necessitated additional support at the rear of the locomotive. In general, the available tractive effort differed little from that of the Pacific, but the steam-raising ability was increased, giving more power at speed. The 4-6-4 was best suited to high-speed running across flat terrain. Since the type had fewer driving wheels than carrying wheels, a smaller percentage of the locomotive's weight contributed to traction, compared to other types. Like the Pacific, it was well suited for high speed passenger trains, but not for starting heavy freight trains and slogging on long sustained grades, where more pairs of driving wheels are better.
The first 4-6-4 tender locomotive in the world was a four-cylinder compound locomotive, designed by Gaston du Bousquet for the Chemin de Fer du Nord in France in 1911. Since it was designed for the Paris-Saint Petersburg express, it was named the Baltic after the Baltic Sea, which was a logical extension of the naming convention that started with the 4-4-2 Atlantic and 4-6-2 Pacific.
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