Big Boy was the name of the Union Pacific Railroad

Union Pacific Railroad

The Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest railroad network in the United States. James R. Young is president, CEO and Chairman....

's 4000-class 4-8-8-4

4-8-8-4

In the Whyte notation for the classification of steam locomotives by wheel arrangement, a 4-8-8-4 is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel trailing truck.Other equivalent classifications are:...

 articulated steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

s, built between 1941 and 1944 by American Locomotive Company

American Locomotive Company

The American Locomotive Company, often shortened to ALCO or Alco , was a builder of railroad locomotives in the United States.-Early history:...

 (Alco). The 25 Big Boys were the only locomotives to have the 4-8-8-4 wheel arrangement

Wheel arrangement

In rail transport, a wheel arrangement is a system of classifying the way in which wheels are distributed beneath a locomotive.. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and connections, with the adopted notations varying by country...

, which combined two sets of eight driving wheels with a four-wheel leading truck for stability entering curves and a four-wheel trailing truck to support the large firebox.
The Union Pacific Railroad (UP) introduced the Challenger

Union Pacific Challenger

Built between 1936 and 1943, the Challengers were nearly 122 feet long and weighed more than one million pounds. Articulated like their big brother, the Union Pacific Big Boy, the Challengers had a 4-6-6-4 wheel arrangement...

-type (4-6-6-4

4-6-6-4

In Whyte notation, a 4-6-6-4 is a railroad steam locomotive that has four leading wheels followed by six coupled driving wheels, a second set of six driving wheels and four trailing wheels.Other equivalent classifications are:...

) locomotives in 1936 on its main line across Wyoming.