One of the first railroads in the USA to purchase the 4-8-2 “Mountain” wheel arrangement was the Rock Island. In 1913, however, only the two prototypes of the M-50 class were initially delivered. They had square cylinder dimensions measuring 28 by 28 inches and drivers measuring 69 inches or 1,753 mm in diameter.

The First World War had to pass before series production came about. Between 1920 and 1924, 80 more M-50 were built in several batches, some of which were fired with coal and some with oil. They also had larger drivers of 74 inches or 1,880 mm, a larger firebox thanks to thermic syphons and a boiler pressure of 200 instead of 180 psi. Some of the coal-fired machines later received a stoker.

In 1941 seven were sold to the St. Louis Southwestern and rebuilt there. The Rock Island also rebuilt 18 of its locomotives to the M-50a. They got roller bearings on all axles, lighter connecting and coupling rods, disc drivers, a boiler pressure of 220 psi and two inch smaller cylinders that were cast in one piece along with the front frame.