The Union Pacific Big Boy is a type of simple articulated 4-8-8-4 steam locomotive manufactured by the American Locomotive Company (ALCO) between 1941 and 1944 and operated by the Union Pacific Railroad in revenue service until 1962.

The 25 Big Boy locomotives were built to haul freight over the Wasatch mountains between Ogden, Utah and Green River, Wyoming. In the late 1940s, they were reassigned to Cheyenne, Wyoming, where they hauled freight over Sherman Hill to Laramie, Wyoming. They were the only locomotives to use a 4-8-8-4 wheel arrangement: 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.

Today, eight Big Boys survive, with most on static display at museums across the country. One of them, No. 4014, was re-acquired by Union Pacific and rebuilt to operating condition in 2019, regaining the title as the largest and most powerful operating steam locomotive in the world.

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