one of the largest and most powerful types of steam locomotives ever built. Designed primarily to handle heavy freight traffic in the United States during World War II, 25 Big Boys were produced from 1941 to 1944 exclusively for the Union Pacific Railroad (Union Pacific Railroad Company). They were gradually replaced by the diesel-electric locomotive; the last one was taken out of regular service in 1959.

The Big Boy along with its tender weighed about 604 tons and measured more than 40 m (132 feet) in length. It had a maximum power capacity of more than 6,000 horsepower and could exert 135,400 pounds of tractive force. The locomotive could haul a 3,600-ton train unassisted up a 1.14 percent grade, such as was found on a stretch of Union Pacific track in the Wasatch Mountains. It could achieve a speed of 112 km (70 miles) per hour pulling freight on level track.

The Big Boy had an articulated design; the frame of the front engine was hinge-connected to the rear engine under a single boiler. Its wheel arrangement was designated 4-8-8-4—i.e., a set of 4 pilot wheels led a set of 8 coupled driving wheels, which were compounded by another set of 8 coupled drivers, with 4 trailing wheels.