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.

Overview

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the motive power for express passenger trains throughout much of the early to mid-20th century, before either being superseded by larger types in the late 1940s and 1950s, or replaced by electric or diesel-electric locomotives during the 1950s and 1960s.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. 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.