Under the Whyte notation for the classification of steam locomotives, 2-2-2 represents the wheel arrangement of two leading wheels on one axle, two powered driving wheels on one axle, and two trailing wheels on one axle. The wheel arrangement both provided more stability and enabled a larger firebox than the earlier 0-2-2 and 2-2-0 types. This configuration was introduced in 1834 on Robert Stephenson's 'Patentee locomotive' but it was later popularly named Jenny Lind, after the Jenny Lind locomotive which in turn was named after the popular singer. They were also sometimes described as Singles, although this name could be used to describe any kind of locomotive with a single pair of driving wheels.

Other equivalent classifications are:

UIC classification: 1A1 (also known as German classification and Italian classification)
French classification: 111
Turkish classification: 13
Swiss classification: 1/3

"Pioneer" Locomotive

Replica of 'Le Belge' 1835

The original "Jenny Lind" locomotive, 1847

Great Western Railway North Star at Swindon

Replica of Adler at F├╝rth, May 2008

A SINGLE DRIVER EXPRESS ENGINE designed by J. Holden for the Great Eastern Railway. The driving wheels were 7 ft diameter and the cylinders 18 in by 24 in. The earlier engines of this class had a boiler pressure of 140 lb; in later engines the pressure was 160 lb. The total heating surface was 1,230 sq ft, and the grate area 17.9 sq ft. The weight, without tender, was 40 tons 3 cwt in working order.