4-6-2

4-6-2 (Pacific)

Front of locomotive at left

New Zealand’s Q class of 1901, the first true Pacific locomotive

Equivalent classifications UIC class 2C1 (refined to 2′C′1′ or 2′C1′) French class 231 Turkish class 36 Swiss class 3/6 Russian class 2-3-1

First known tank engine version First use 1896 Country Australia Locomotive Q class Railway Western Australian Government Railways

First known tender engine version First use 1887 Country United States of America Railway Lehigh Valley Railroad Evolved from 4-6-0 Benefits Experimental with Strong's firebox

First known "True type" version First use 1901 Country New Zealand Locomotive Q class Railway New Zealand Railways Dept. Designer A.L. Beattie Builder Baldwin Locomotive Works Evolved from 4-4-2, 4-6-0 & 2-6-2 Benefits Wide and deep firebox behind coupled wheels

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.

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the motive power for express passenger trains throughout much of the early to mid-20th century, before either being superseded by larger types in the late 1940s and 1950s, or replaced by electric or diesel-electric locomotives during the 1950s and 1960s.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