The Great Northern took delivery of its first fifteen Mountain (4-8-2) type locomotives from the Lima Locomotive Works in 1914 (#1750-#1764). Designated Class P-1 and originally intended to haul passenger trains, they proved too slow on the railroad's 2.2% grades over the Cascades and were soon transferred to freight service. In 1928, they were rebuilt as 2-10-2 Santa Fe types.
The GN took delivery of another twenty-eight Mountain type locomotives from Baldwin in 1923 (#2500-#2527). Designated Class P-2, they were considerably larger and heavier than the P-1s. Designed to haul the crack Minneapolis, MN-Seattle, WA, Oriental Limited, ten were coal burners, including #2523, and handled the Oriental east of Cut Bank, MT. The other eighteen were oil burners and operated further west.
The locomotives could haul ten to twelve heavyweight passenger cars at 18 mph on the 1.8% ruling grade eastbound up Walton Hill just before Marais Pass, MT, and their success led to the inauguration of the GN's Chicago, IL-Seattle, WA, Empire Builder service in 1929, although the P-2s were soon replaced by the heavier and more powerful Northern (4-8-4) type S-2s.