The 2-8-8-0 was known amongst railroads as the Consolidation Mallets as they were considered an extension of the common 2-8-0 Consolidation type locomotives. The locomotive type was originally experimented with by the Santa Fe, but the railroad never received more than one locomotive because of balance issues and a low tractive effort rating on the railroad's steepest grades.
The Great Northern and Union Pacific revised the designs and proceeded to order several of these locomotives. The Union Pacific crews developed the nickname Bullmoose for the locomotives because they had the ability to pull the heaviest of freight trains over the railroads Sherman Hill between Cheyenne and Laramie, Wyoming.
The Union Pacific eventually rostered 70 of these Mallets, either built by its own shops or manufactured by the American Locomotive Company (Alco). They first went into service in 1918. As time went on, the Bullmoose locomotives received several upgrades