A4-8-8-4 in theWhyte notation for the classification ofsteam locomotives bywheel arrangement, is a locomotive with a four-wheelleading truck, two sets of eightdriving wheels, and a four-wheeltrailing truck. Only one model of locomotives has ever used this wheel configuration, and that is commonly known as "Union Pacific Big Boys" after the railroad that operated them.[1]
Other equivalent classifications are:
The equivalent UIC classification is refined to (2′D)D2′ for simplearticulated locomotives.
A similar wheel arrangement exists forGarratt locomotives, but is referred to as4-8-0+0-8-4 since both engine units swivel.
4-8-8-4 Big Boys were only produced for theUnion Pacific Railroad. Twenty-five such engines were built between 1941 and 1944, numbered 4000 to 4024. Eight of these locomotives survive, seven of which are on static public display at various sites in theUnited States. Union Pacific announced plans in August 2013[2] to restoreNo. 4014 to operation for use in mainline excursion service. No. 4014 underwent restoration between 2016 and 2019, and completed its first post-restoration test run on May 2, 2019.[3][4]
The UP Big Boys were an expansion of the4-6-6-4 "Challenger" type articulated locomotive. Adding four driving wheels increased the pulling power of the locomotive and reduced the need for helper locomotives over steep grades. Although their wheels were an inch (25.4 mm) smaller than those of the Challengers, they were still able to sustain similar high speeds.
Other American railroads considered buying 4-8-8-4s, including theWestern Pacific Railroad, which already rostered large 2-8-8-2s and 4-6-6-4s, butdiesel locomotives were gaining popularity and soon were able to displace these Big Boy locomotives.
 | This steam locomotive-related article is astub. You can help Wikipedia byexpanding it. |