The 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 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 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.