The Union Pacific 4000 class, otherwise known as the "Big Boy," is a class of 25 4-8-8-4 (meaning it had 4 guide wheels, 2 sets of 8 drive wheels, and 4 trailing wheels to support the firebox and cab) articulated steam locomotive, built from 1941-1944 to haul heavy freight trains on the UP's mountainous territory, mainly Utah and Wyoming. While some sites refer to the big boys as the biggest steam trains ever, this isn't entirely true, but they are in the top 1% of steam engines in strength and size They were 133 feet long, weighed 1.2 million pounds, and could haul 3,500 ton trains unassisted up the road's steepest grades, or even heavier ones on flatter terrain, and were surprisingly fast for an engine their size, capable of 75 mph. The class was out of service by 1962, and 8 have been preserved for museums, one of which, 4014, was bought back by the Union Pacific, and restored to operation, and as of 2025, still hauls excursion trains for the U.P
