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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. 4-8-8-4s are commonly known as "Big Boys". Other equivalent classifications are: UIC classification: 2DD2 French classification: 240+042 Turkish classification: 46+46 Swiss classification: 4/6+4/6 The equivalent UIC classification is refined to D2′ for Mallet locomotives. 4-8-8-4 Big Boys were only produced for the Union Pacific Railroad. Twenty-five such engines were built between 1941 and 1944, numbered 4000 to 4024. Eight of these survive, all on static public display at various sites in the USA. Union Pacific has announced plans to restore No. 4014 to operation for use in mainline excursion service. No. 4014 is expected to be operational by 2019. The UP Big Boys were an expansion of the 4-6-6-4 "Challenger" type articulated locomotive. Although their wheels were an inch smaller than those of the Challengers, they were still able to sustain high speeds.

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