Mainstays of the Rock Island's premier varnish in pre-diesel days were the 60 class M-50 4-8-2s that the American Locomotive Company constructed in five groups between 1920 and 1929. A mix of coal-burners and oil-burners, these Mountain types as delivered had 74-inch drivers, 28x28-inch cylinders, and 200 p.s.i. of boiler pressure. They developed 50,430 pounds of tractive effort, and as coal-burners weighed 260,250 pounds. They had 63 square feet of grate area, 4563 square feet of evaporative heating surface, and 1338 square feet of superheating surface.

No. 4049, shown here, was delivered in 1927 and was one of a group of eighteen members of the class that the Rock Island's Silvis Shops modernized between 1939 and 1942 with roller bearings on all axles, light-weight rods, and a new cast cylinder and forward frame. The cylinder diameter was reduced to 26 inches and the boiler pressure raised to 220 p.s.i., resulting in a slight reduction in tractive effort. As rebuilt, this group was reclassified to M-50a. No information is forthcoming concerning the date and location of this photo, and the photographer's identity is likewise lost to us.