T. D. Judah, a locomotive rebuilt as a 4-2-2 by Central Pacific Railroad.
(Unknown, Public domain, via Wikimedia Commons)
4-2-2 SINGLE WHEEL ARRANGEMENT
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
- 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.
Diagram of the 4-2-2 Wheel Arrangement. Front of locomotive on left.
(Gwernol, Public domain, via Wikimedia Commons)
History
The 4-2-2 configuration offered designers eight wheels to spread the weight of a larger locomotive, but prior to the bogies (invented in the 1830s) becoming popular, 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 a 4-2-2, but the company quickly reverted to the more common 2-2-2 configuration.
US developments
The T. D. Judah locomotive was built as a 4-2-4 by the Cooke Locomotive and Machine Works in 1863. It was purchased for use on the Central Pacific Railroad and was rebuilt as a 4-2-2 in 1872.
By 1900, typical loads on express trains had grown beyond the capabilities of 4-2-2 locomotives and the configuration was superseded by the 4-4-2.