National Railroad Museum, Green Bay Wi
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4-8-8-4 Union Pacific "Big Boy" 
 
Big Boy was the name of the Union Pacific Railroad's 4000-class 4-8-8-4 articulated steam locomotives, built between 1941 and 1944 by American Locomotive Company (Alco). The 25 Big Boys were the only locomotives to have the 4-8-8-4 wheel arrangement, which combined two sets of eight driving wheels with a four-wheel leading truck for stability entering curves and a four-wheel trailing truck to support the large firebox.