Union Pacific 4014 is a preserved 4884-1 class 4-8-8-4 "Big Boy" type steam locomotive owned and operated by the Union Pacific as part of its heritage fleet. Built in November 1941 by the American Locomotive Company in Schenectady, New York, it was assigned to haul heavy freight trains in the Wasatch mountain range. The locomotive was retired from revenue service on December 7, 1961, and was donated to the Railway & Locomotive Historical Society; thereafter, it was displayed in Fairplex at the RailGiants Train Museum in Pomona, California.
In 2013, UP re-acquired the locomotive and launched a restoration project at its Steam Shop in Cheyenne, Wyoming. In May 2019, No. 4014 moved under its own power after sitting dormant for almost six decades, becoming the world's largest operational steam locomotive and the only operating Big Boy locomotive of the eight that remain in existence. It now operates in excursion service for the UP steam program. No. 4014 became the first mainline steam locomotive to be equipped with the positive train control system in 2021.