4-8-8-4
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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 significant 4-8-8-4 locomotives built 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. Large diameter driving wheels were fitted to allow higher speed freight runs in terrain with grades. Adding two more sets of driving wheels increased the pulling power of the locomotive and lessened the need for helper locomotives over steep grades for a given tonnage train.
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. They have since become iconic to the Union Pacific Railroad.
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