In Whyte notation, a 4-2-4 is a steam locomotive that has a four-wheel leading truck, one powered driving axle and a four-wheel unpowered trailing truck.
Other equivalent classifications are:
UIC classification: 2A2 (also known as German classification and Italian classification)
French classification: 212
Turkish classification: 15
Swiss classification: 1/5
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This most unusual wheel arrangement was limited to tank locomotives.
The type appears to have first been used on 14 locomotives supplied to the broad gauge Bristol and Exeter Railway in 1853. They were designed by James Pearson, and featured single large flangeless driving wheels and two supporting bogies. The water was carried in both well and back tanks. Three classes were distinguished by the size of driving wheel; the original 9-foot-diameter (2.7 m) wheels were replaced by smaller ones on later designs. The type was also used on an experimental locomotive by William Dean of the Great Western Railway 1881. It did little work and was prone to derailment. It was rebuilt as a 2-2-2 tender locomotive in 1884.
Dugald Drummond of the London and South Western Railway built a 4-2-4T LSWR F9 class combined locomotive and inspection saloon in 1899. It was little used after Drummond's death in 1912.[1]
C. P. Huntington was one of three identical 4-2-4 locomotives which were the first locomotives purchased by Southern Pacific Railroad in 1863, for use on light commuter services in the Sacramento area. They were soon deemed too light and lacking in adhesion for most short trains, due to their single driving wheel axle that did not have the full weight of the engine's rear due to the trailing truck. As a result they were not used frequently except when absolutely needed. Also the short water tank on the Forney type frame prevented the locomotives from traveling any moderate distance without consuming all of the water. A sister engine, T. D. Judah was built by the Cooke Locomotive Works in 1863 for a railroad that was unable to pay for it and purchased by the Central Pacific Railroad. It was rebuilt as a 4-2-2 in 1872.
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