4-6-2

Under the Whyte notation for the classification of steam locomotives, 4-6-2 represents the wheel arrangement of four leading wheels on two axles, six powered and coupled driving wheels on three axles and two trailing wheels on one axle. The 4-6-2 locomotive became almost globally known as a Pacific type after a locomotive built by the Baldwin Locomotive Works in Philadelphia was shipped across the Pacific Ocean to New Zealand.

The introduction to New Zealand of the 4-6-2 design in 1901 has been described as "a veritable milestone in locomotive progress". On many railways worldwide, Pacific steam locomotives provided the motive power for express passenger trains throughout much of the early to mid-20th century, before either being superseded by larger types in the late 1940s and 1950s, or replaced by electric or diesel-electric locomotives during the 1950s and 1960s. Nevertheless, new Pacific designs continued to be built until the mid-1950s.

The type is generally considered to be an enlargement of the 4-4-2 Atlantic type, although its prototype had a direct relationship to the 4-6-0 Ten-wheeler and 2-6-2 Prairie, effectively being a combination of the two types. The success of the type can be attributed to a combination of its four-wheel leading truck which provided better stability at speed than a 2-6-2 Prairie, the six driving wheels which allowed for a larger boiler and the application of more tractive effort than the earlier 4-4-2 Atlantic, and the two-wheel trailing truck, first used on the New Zealand 2-6-2 Prairie of 1885. This permitted the firebox to be located behind the high driving wheels and thereby allowed it to be both wide and deep, unlike the 4-6-0 Ten-wheeler which had either a narrow and deep firebox between the driving wheels or a wide and shallow one above.

The type is well-suited to high speed running.^{[citation needed]} The world speed record for steam traction of 126 miles per hour (203 kilometres per hour) has been held by a British Pacific locomotive, the Mallard, since 3 July 1938.

The two earliest 4-6-2 locomotives, both created in the United States, were experimental designs which were not perpetuated. In 1887, the Lehigh Valley Railroad experimented with a 4-6-0 Ten-wheeler design with a Strong's patent firebox, a cylindrical device behind the cab which required an extension of the frame and the addition of two trailing wheels to support it. In 1889, the Chicago, Milwaukee & St Paul Railway rebuilt a conventional 4-6-0 with trailing wheels as a means of reducing its axle load.

In 1896, six Q class 4-6-2 tank locomotives were introduced on the Western Australian Government Railways.

The first true Pacific, designed as such with a large firebox aft of the coupled wheels, was ordered in 1901 by the New Zealand Railways Department (NZR) from the Baldwin Locomotive Works of Philadelphia, Pennsylvania. The NZR Chief Mechanical Engineer, Alfred Beattie, ordered thirteen new Q class locomotives with a sufficiently large Wootten firebox to efficiently burn poor grade lignite coal from eastern South Island mines. Even before they had completed the order from New Zealand, the Baldwin engineers realised the advantages of this new type, and incorporated it into standard designs for other customers. The design was soon widely adopted by designers throughout the world.

There are different opinions concerning the origin of the name Pacific. The design was a natural enlargement of the existing Baldwin 4-4-2 Atlantic type, but the type name may also be in recognition of the fact that a New Zealand designer had first proposed it. Usually, however, new wheel arrangements were named for, or named by, the railroad which first used the type in the United States. In the case of the Pacific, that was the Missouri Pacific Railroad in 1902.