4-6-4

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 combines 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 has fewer driving wheels than carrying wheels, a smaller percentage of the locomotive's weight contributes to traction, compared to other types. Like the Pacific, it was well suited for high speed passenger trains, but for starting heavy freight trains and slogging on long sustained grades, more pairs of driving wheels are better.