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.
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 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.
UK developments
The London and North Western Railway No. 3020 Cornwall was built as 4-2-2 at Crewe Works 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 & 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.
The first 4-2-2 to have a bogie was built by Archibald Sturrock of the Great Northern Railway (GNR) in 1853. This had flangeless driving wheels, and was only moderately successful, having a tendency to derail. By the 1870s, improved design of bogies giving more flexibility enabled designers to create fast standard gauge express passenger locomotives of this type. On the GNR, Patrick Stirling built 53 examples with outside cylinders at Doncaster Works between 1870 and 1895, for use on the East Coast Main Line between London King's Cross and York. They ran at an average speed of more than 60 mph during the race to the north, and were called eight-footers because of the driving wheel, that was more than 8 ft. in diameter. Stirling's successor Henry Ivatt built a further twelve singles between 1898 and 1901 before moving on to larger 4-4-2 designs.
The attraction of the 'single' (4-2-2 or otherwise) was that, thanks to the 'gearing' effect of the large single driving wheels, a locomotive could obtain high speeds while the operating speed of the engine's pistons and valve gear remained relatively low. This was important because of the relatively unsophisticated lubrication systems available at the time, with many of the parts of the engine's motion requiring to be 'oiled round' with the locomotive stationary before and after a run, or on longer trips by one of the crew taking to the running board to oil the required parts while on the move. Low piston speed also meant that the steam demand on the boiler would be relatively low, allowing a smaller, lighter boiler.