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Big Boy is the popular name of the American Locomotive Company 4000-class 4-8-8-4 articulated, coal-fired, steam locomotives manufactured between 1941 and 1944 and operated by the Union Pacific Railroad until 1959. The Big Boy fleet totaled twenty five locomotives that were used primarily in the Wyoming Division to haul freight over the Wasatch range between Green River, Wyoming and Ogden, Utah. The Big Boy locomotives were the only locomotives to use a 4-8-8-4 wheel arrangement, consisting of a four-wheel leading truck for stability entering curves, two sets of eight driving wheels and a four-wheel trailing truck to support the large firebox.
According to Ed Dickens Jr., Union Pacific's senior manager of heritage operations, the legendary name was coined by an unknown worker who scrawled 'Big Boy' in chalk on the front of one of the engines as it was being built. It became the nickname thereafter.