The only 4-8-2 Mountain locomotives ever operated by the Union Pacific were the classes MT-1 and MT-2, which were nearly identical apart from their firing method. The first 40 MT-1 were built in 1922 by ALCO, followed by 15 more in 1923 which weighed 3,000 pounds less. These were all coal-fired and had a tender that held 12,000 gallons of water and 20 tons of coal. Five MT-2 delivered in 1924 were oil-fired and had tenders with 15,000 gallons of water and 5,480 gallons of oil.

These locomotives were used to haul important passenger trains over long distances. They ran distances of 500 to 700 miles without major stops. Later, many were fitted with a partial casing. Only 7002 was totally streamlined and got roller bearings on all axles and the rods. It was used to haul the all-Pullman “Forty-Niner” when it was found out that early diesels could not keep up the schedule between Chicago and San Francisco. By 1956, all MT-1 and MT-2 locomotives had been scrapped.