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.

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.

The first 4-6-4 in the United States of America, the J-1 of the New York Central Railroad, was built in 1927 to the railroad’s design by the American Locomotive Company (ALCO). There the type was named the Hudson after the Hudson River.

The world speed record for steam locomotives was held by a 4-6-4 at least twice. In 1934 the Milwaukee Road’s class F6 no. 6402 reached 103.5 miles per hour (166.6 kilometres per hour), and in 1936 the German class 05.002 reached 124.5 miles per hour (200.4 kilometres per hour). That record was broken by the British 4-6-2 Pacific no. 4468 "Mallard" on 3 July 1938, when it reached 126 miles per hour (203 kilometres per hour), still the world speed record for steam traction.