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).
In the United States and Britain, the 4-6-2 locomotive type was named the Pacific, according to some sources after some locomotives built for the Missouri 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 US and Canadian railroads. The largest user was the Pennsylvania, who had a total of 697 Pacifics, including 425 class K4s, the largest single class of locomotive ever built in the world.
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, permitting 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 fatal 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.