4-6-4

4-6-4, under the Whyte notation for the classification of locomotives, 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 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 4-6-2, 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 4-6-2, 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 Chemins 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 and 4-6-2.

The first 4-6-4 in the United States of America, J-1a #5200 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. They are also designed to pull 16-18 passenger cars in passenger service.

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 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.

The 4-6-4T was also a fairly common wheel arrangement for passenger tank locomotives. As such, it was essentially the tank locomotive equivalent of a 4-6-0 tender locomotive, with water tanks and a coal bunker supported by four trailing wheels instead of in a tender. In New Zealand, some 4-6-4T locomotives (the Wab class) were tank versions of 4-6-2 locomotives (of the Ab class).

The first known 4-6-4 tank locomotive was rebuilt from a Natal Government Railways (NGR) K&S Class 4-6-0T which was modified in 1896 to enable it to run equally well in either direction on the Natal South Coast line, where no turning facilities were available at the time. This sole locomotive later became the Class C2 on the South African Railways (SAR). The first known locomotive class to be designed with a 4-6-4T wheel arrangement, the NGR's Class F tank locomotive, was based on this modified locomotive and built by Neilson, Reid & Company in 1902. These became the Class E on the SAR in 1912.

One streamlined 4-6-4T was built for the Deutsche Reichsbahn in 1935.