4-2-2

Under the Whyte notation for the classification of steam locomotives, 4-2-2 represents the wheel arrangement of four leading wheels on two axles, two powered driving wheels on one axle, and two trailing wheels on one axle.

Other equivalent classifications are:
UIC classification: 2A1 (also known as German classification and Italian classification)
French classification: 211
Turkish classification: 14
Swiss classification: 1/4

Like other steam locomotive types with single pairs of driving wheels, they were also known as singles.

The 4-2-2 configuration offered designers eight wheels to spread the weight of a larger locomotive, but prior to the introduction of bogies, created a long rigid wheelbase with limited adhesion. As a result, the type was relatively rare until the 1870s. The first steam locomotive made by Borsig of Berlin in 1841, the Borsig No 1, was an 4-2-2, but the company quickly reverted to the more common 2-2-2 configuration.

The London and North Western Railway No. 3020 Cornwall was built as 4-2-2 at Crewe in 1847, but was extensively rebuilt, and converted to a 2-2-2 in 1858.

The one area where the type proved to be useful was on broad gauge locomotives, where sharp bends were less of an issue. Daniel Gooch built 29 examples of his Iron Duke express locomotive class for the Great Western Railway between 1847 and 1855. They had an 8 ft diameter driving wheel size. Twenty examples of a similar design were built for the Bristol and Exeter Railway after 1849, by Stothert and Slaughter in Bristol. Because both sets of leading wheels are mounted independently in the frames in these classes, they are sometimes described as (2-2)-2-2 rather than 4-2-2.

...
Wikipedia