4-6-2
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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.
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 US 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
All German 4-6-2's were express passenger locomotives with large driving wheels. They were the BR 01, BR 02, BR 03, BR 04 (experimental), BR 10 and the BR 18 with its many subclasses. Some were streamlined.
Austria
The only application of this wheel arrangement in Austria was in tank locomotives of class 629, built from 1913 on. This highly successful type remained in service until 1975.
Finland
22 Pacifics were constructed in Finland, by Tampella and Lokomo between 1937 and 1957. They were the largest passenger locomotives ever built, or used, in Finland.