After the P-33's 25 inch diameter cylinders were too large for the boiler, they had to be downsized to 23.5 inches. By increasing the grate area from 45 to 63 square feet, the P-40 class was born. They could now even enlarge the cylinders to 25.5 inches and still have enough steam. Like the P-33, it was oil fired.

A passenger train consisting of twelve cars and weighing 900 short tons was chosen to determine the pulling power. A one percent steep, six-mile incline was begun at 35 mph and averaged 27 mph without the boiler pressure dropping. In contrast to the previous locomotives, it was not necessary to subsequently install a booster or thermic siphons. 

In contrast to the P-33, the P-40 was not built by ALCO's Schenectady plant, but by ALCO-Brooks. With 30 locomotives, however, a smaller number was produced than with its predecessor. The first were retired in the late 1930s, the last in 1952.

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.

The first 65 class R-65 Northerns of the Rock Island were delivered by ALCO-Schenectady in 1929 and 1930. They had a driver diameter of 69 inches and were primarily intended for service with freight trains. Some later received 74 inch wheels to be more suitable for passenger trains and were renamed the R-67 class as a result. In the years 1944 and 1946 further R-67b were built, which had larger wheels from the start. Although they had a higher boiler pressure and a larger firebox, they had a smaller tube heating area and a significantly smaller superheater. The 1944 batch were oil fired while the rest were coal fired.

The class C-43 of the Rock Island didn't technically designate a series of similar locomotives, but only stood for Consolidations with a tractive effort of around 43,000 pounds. 83 locomotives with the numbers 1701 to 1783 were built by Baldwin in 1906. With drivers of 63 inches and cylinders of 23 by 30 inches, they initially only produced a tractive effort of 39,600 pounds. Only after they got larger cylinders when being superheated starting in 1918, they produced 43,100 pounds. ALCO-Brooks built numbers 1901 to 1930 in 1907 which were similar to the first batch.

Also in 1907, ALCO-Schenectady built numbers 2100 to 2144 which had drivers of only 57 inches and a higher boiler pressure and produced 43,300 pounds even with cylinders of only 22 by 30 inches. They had a firebox that was somewhat larger than that of the earlier Consolidations, but with roughly the same grate area. Most were later superheated and were fitted with thermic syphons which increased the grate area from 183 to 240 square feet. Like the earlier Consolidations, they were retired between 1936 and 1953.