In the Whyte notation for the classification of steam locomotives by wheel arrangement, a 4-8-8-4 is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel trailing truck.
Other equivalent classifications are:
UIC classification: 2DD2 (also known as German classification and Italian classification)
French classification: 240+042
Turkish classification: 46+46
Swiss classification: 4/6+4/6
The equivalent UIC classification is refined to (2′D)D2′ for Mallet locomotives.
The only 4-8-8-4 steam locomotives were the Union Pacific 4000 "Big Boy" Class. The UP Big Boys were an expansion of the 4-6-6-4 "Challenger" type articulated locomotive. Although their wheels were an inch (25 mm) smaller than those of the Challengers, they were still able to sustain high speeds. Adding four driving wheels increased the pulling power of the locomotive and reduced the need for helper locomotives over steep grades.
Other American railroads considered buying 4-8-8-4s, including the Western Pacific Railroad, which already rostered large 2-8-8-2s and 4-6-6-4s, but diesel locomotives were gaining popularity and soon were able to displace these monster locomotives.
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