4-6-2

Under the Whyte notation for the classification of steam locomotives, a 4-6-2 Locomotive has four leading wheels (generally arranged in a leading truck), six coupled driving wheels and two trailing wheels (often but not always in a trailing truck).

The equivalent UIC classification is 2'C1'.

In the United States and Britain, the 4-6-2 locomotive type was named the Pacific, according to some sources after some early locomotives of this type built for the Missouri Pacific Railroad. A famous 4-6-2 locomotive was the LNER A4 class 4468 Mallard.

The term Pacific comes from the fact that the New Zealand Railways (NZR) were the first to purchase and operate a proper 4-6-2 wheel arrangement in 1901, with the order of 13 being placed with Baldwin. The reason for the design was for a locomotive with a large Firebox used for burning poor Lignite Coal from a South Island coal mine. So it was the New Zealand Railways Chief Mechanical Engineer, A. W. Beattie, who ordered the first true Pacific. There are some known examples of Pacifics prior to 1901, but these were rebuilds or 4-6-0s and thereby not proper as built Pacifics. In 1915 the NZR built the (reputably) first steam locomotive to develop 100 hp for 100 lb of engine weight. It was a New Zealand built and designed Pacific.

Pacifics were the predominant steam passenger power in America in the twentieth century. Few railroads did not roster 4-6-2 locomotives as premier passenger power, although they were supplanted in top-flight service on many roads later on by larger 4-6-4 "Hudson", 4-8-2 "Mountain" or 4-8-4 "Northern" locomotives as train weights increased.

Approximately 7000 locomotives of this wheel arrangement were produced for U.S. and Canadian railroads. The largest user was the Pennsylvania Railroad, which had a total of 697 Pacifics, including 425 class K4s, the largest single class of locomotive ever built in the United States (they claimed worldwide, but this is unlikely to be true given huge fleets of identical German, Russian, and Chinese locomotives).

The success of the 4-6-2 design can be attributed to the presence of a 4-wheel leading truck, which made for stability at speed; six driving wheels which allowed for the application of more power compared to the earlier 4-4-2 "Atlantic" design; and a two-wheel trailing truck which permitted the Firebox to be behind the high driving wheels, allowing it to be both wide and deep. On a locomotive without a trailing truck such as a 4-6-0 "Ten Wheeler", the designer is caught in a dilemma; the firebox can either fit between the driving wheels and be narrow and deep, or it can fit above the driving wheels and be wide and shallow.

The Pacific was further developed into the 4-6-4 "Hudson", with a 4-wheel trailing truck permitting a still larger firebox, and into the 4-8-2 "Mountain", with an extra pair of driving wheels giving more traction, and thus able to transfer more power.

Germany

Austria

Finland