The 2-8-8-0 Mallet locomotives of the Union Pacific and its subsidiaries were known as classes MC-2 to MC-6. All batches were characterized by 57 inch drivers and cylinders with diameters of 26 and 41 inches and were built by ALCO-Brooks or ALCO-Schenectady. Their main area of operation were the grades in Wyoming, like the famous Sherman Hill. The UP called them “Bull Mooses”, a term that's sometimes used for this wheel arrangement in general.

Each of the five sub classes designates a batch with only minor changes compared to each other. The first one were 16 MC-2 built in 1918 by Schenectady, while all later batches were built by Brooks. These were 19 MC-3 in 1920 and ten MC-4 in 1923, the latter having somewhat differing dimensions in the boiler. The 20 MC-6 built in 1924 had fewer tubes, but a larger firebox heating surface thanks to thermic syphons.

Between 1936 and 1937, most were converted to burn oil, with their running numbers changed from the 3600 to the 3500 range. A bigger rebuild took place between 1937 and 1946, when they were simpled and the individual subclasses were renamed to SA-C-2 to SA-C-6. They got smaller cylinders with a diameter of only 23 inches and even the stroke was reduced. Even though the driver diameter was increased by two inches, the resulting tractive effort now was bigger than it had been previously in compound mode.

As early as in the late twenties, the introduction of the faster and more powerful class 9000 4-12-2 meant that the Bull Mooses were not used anymore for fast freights. When the Challengers were introduced, they were only used as helpers on the grades. In this role, they were retired between 1947 and 1954.