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Under the Whyte notation for the classification of steam locomotives, a 4-6-4 locomotive has four leading wheels (generally arranged in a leading truck), six coupled driving wheels and four trailing wheels (often but not always in a trailing truck).

The 4-6-4 is best seen as combining the basic nature of the 4-6-2 'Pacific' type with an improved boiler and larger firebox that required extra support at the rear of the locomotive. Generally the available tractive effort was little different from that of the Pacific, but steam-raising ability was increased, giving more power at speed.

4-6-4s were best suited to high-speed running across flat country. The type has fewer driving wheels than carrying wheels and thus a smaller percentage of the locomotive's weight is available for traction compared to other types. For starting heavy trains and slogging on gradients, a 4-6-4 really needs a booster engine, but for sustained long grades, more pairs of driving wheels are better.

4-6-4 was also a fairly common wheel arrangement for a passenger tank locomotive—basically, a double-ended Pacific, able to travel in both directions with equal facility.

Table of contents
1 United States
2 Japan
3 France
4 United Kingdom
5 Germany

United States

The first 4-6-4 tender locomotive was built in 1927 by ALCO for the New York Central Railroad, and to the NYC's design. The locomotive proved very successful and was named the Hudson type after the Hudson River. The NYC acquired 275 Hudsons, the largest fleet in North America.

The Milwaukee Road would have produced the first 4-6-4; its design was earlier than the NYC's, but financial constraints delayed the project, and Milwaukee's locomotives emerged later. The Milwaukee called them Baltics and that name was fairly widely used also. The initial order of 14 class F-6 locomotives was joined by 8 more of class F-6a a year later in 1931, and in 1938 the road acquired 6 streamlined Baltics with shrouds designed by noted industrial designer Otto Kuhler. These took over the Milwaukee's crack Hiawatha expresses from the A-1 class 4-4-2 Atlantics, and were among the fastest steam locomotives of all time. Schedules of many of these trains required extended running substantially above 100 mph.

The second largest buyer of the type in North America was the Canadian Pacific Railway, who bought 65. They were highly successful in improving service and journey times on the CPR's transcontinental routes. The newer CPR Hudsons were called 'Royal Hudsons' and were semi-streamlined. Royal permission was given for these locomotives to bear the Royal crown and arms after such a locomotive hauled King George VI across Canada in 1939.

20 railroads in North America owned 4-6-4s; these included, as well as the foregoing, the Santa Fe, Baltimore & Ohio, Boston & Albany, Big Four, Canadian National, Chesapeake & Ohio, Burlington, Chicago & North Western, Lackawanna, Illinois Central, Maine Central, Michigan Central Railroad, National Railway of Mexico, New Haven, Nickel Plate, Frisco, and Wabash.


The Japanese National Railways built three classes of rather advanced, American style 3'6" (1,067 mm) gauge Hudsons, classes C60 (47 built), C61 (33 built) and C62 (49 built). The C60 and C61 were smaller and the C62 was a larger locomotive, filling the small Japanese loading gauge. All were officially rebuilt from earlier locomotives of different arrangement, but it is believed that this was for accounting purposes rather than any real cost saving; the parts re-used appear to have been minimal. They were all equipped with disk drivers and much in the way of American-style appliances, although they had British-style smokebox doors.


United Kingdom

A number of 4-6-4 tank locomotives were built for various British railway companies. The London, Brighton & South Coast Railway built seven Remembrance Class tank locomotives between 1914 and 1922; these high-speed tank locomotives hauled the famous Southern Belle until electrification in 1933, after which they were converted into N15X class 4-6-0 tender locomotives, remaining in service until 1957.

The only 4-6-4 tender locomotive in Great Britain was LNER No. 10000, built in 1930 as an experimental high-pressure compound locomotive with an experimental water-tube boiler, and known as the "hush-hush" locomotive on account of the great secrecy with which it was built. The experiment proved much less successful than hoped, and in 1936 it was rebuilt along the lines of a streamlined LNER A4 Pacific, though it retained its unique wheel arrangement. It was the only locomotive of class W1. Its trailing wheels were arranged uniquely; instead of being in one 4-wheel trailing truck, the first pair were instead a Cartazzi axle, as typical of LNER Pacific practise, being mounted in a rigid frame but allowed sideways deflection against a centering force. The second pair were in a two-wheel trailing truck. Thus some claim No. 10000 was not a true 4-6-4 but more of a 4-6-2-2. After its rebuild, the W1 was easily distinguishable from an A4 at a glance, without looking for the extra trailing wheels, by the fact that it was never named; it was therefore known to train spotters as "the no-name streak".


In addition to a number of 4-6-4 tank locomotives, three 4-6-4 tender locomotives were built in 1935. Classified as BR 05, they were designed for high speed running; they were 3-cylinder locomotives, with giant 90½ inch driving wheels and powerful clasp brakes on all wheels. The first two locomotives, 05 001 and 05 002 were conventional locomotives, but the third, 05 003 was built as a cab forward, burning pulverised coal. All were built streamlined, in shrouds that covered the locomotives almost to the railhead. In 1936, 05 002 set a world speed record of 124.5 mph (199 km/h) which soon afterwards was beaten by the LNER's famous Mallard.

All three survived World War II and were rebuilt as conventional, unstreamlined locomotives in 1950 with new boilers, in which form they worked until 1957 when electric locomotives took over the high-speed routes. The first locomotive, 05 001, was restored to its original streamlined configuration and placed into a museum in 1961.