These 20 Ten-Wheelers were built by ALCO-Rogers for the Chicago Southern Railroad between 1905 and 1907. When this company went bankrupt, 14 locomotives had not yet been delivered. Ten of these were purchased by Northern Pacific and designated class S-10. The remaining four went to other customers.

Eight of the Southern Pacific were retired between 1928 and 1933. Only 321 and 328 remained in use on branch lines. While 321 was scrapped in 1946, 328 ran for the last time in 1948. After being restored by the Minnesota Transportation Museum, it was operational from 1981 to 2001. Today it can be found at the Jackson Street Roundhouse in St. Paul, Minnesota.

The Northern Pacific was the first US railroad to purchase a significant number of Mikados. These machines grouped under the class W were 55 locomotives of the subclass WA without a combustion chamber and 105 WB with a combustion chamber. Some of these were built as compound locomotives and later rebuilt to the simple variant. 40 class W-1 locomotives followed in 1910, which differed mainly in that they had Walschaerts valve gear in contrast to the Stephenson valve gear of their predecessors. Most of the locomotives were superheated from 1912, and the boiler pressure was increased to 200 psi with thicker boiler plates.

The W-3 class was created in 1913 as an enlarged successor to the W class. It was factory fitted with a superheater and, from the second batch, also with a combustion chamber. This made it possible to increase the train weight by almost 30 percent. 135 W-3s were built in 1913 and 1917 to 1920. Ten made it to the Spokane, Portland & Seattle where they became Class O-3. In 1922 another 25 W-5 followed, which were almost identical but had a higher boiler pressure.

The first Challenger locomotives, i.e. the wheel arrangement 4-6-6-4, were built in 1936. In addition to the well-known Challenger of the Union Pacific, this was the Z-6 class of the Great Northern. Since these had to burn lignite, the firebox had to be of enormous size to achieve the required output. The grate alone was 16 feet long, the entire firebox including the combustion chamber was 20 feet 6 inches long, 9 feet 6 inches wide and up to 7 feet 6 inches high. This made a grate area of 152.3 square feet and made the trailing bogie necessary to be able to carry the load.

Z-6 No. 5106 in September 1952 in Spokane, Washington

D. W. Eldridge / collection Taylor Rush

21 examples of the Z-6 were delivered by ALCO in Schenectady in 1936 and 1937. The first four examples still had friction bearings on the axles and were approximately 4,500 pounds or two tons heavier than the rest with Timken roller bearings, the weights of which are given in the table below. With a coupling wheel diameter of 69 inches, they could also pull heavy passenger trains if necessary.

Z-7 No. 5121 waiting for it's clear signal with a long freight train in November 1941 somewhere in Montana

W. R. McGee / collection Taylor Rush

In 1941, six heavier examples followed with slightly different boiler dimensions, one inch larger drivers and a boiler pressure increased by 10 psi, which formed the Z-7 class. Instead of the six-axle tender with a water capacity of 22,000 gallons, seven-axle 25,000-gallon tenders were used. In the years 1943 and 1944, 20 almost identical Z-8 were manufactured, but because of the wartime conditions, many of the high-strength steels had to be replaced with conventional, heavier steels. This added about 1,000 pounds to the mass.

The Z-8s were the last steam locomotives procured by the Northern Pacific. Because they were fired with lignite, they consumed gigantic amounts of fuel. On the 65 mile route from Townsend to Bozeman, Montana with a constant incline, there were reports of coal consumption of 831 pounds per mile. The relatively young locomotives were retired between 1954 and 1959. The Spokane, Portland & Seattle Railway procured almost identical locomotives, namely six Z-6 and two Z-8 with oil firing.

In 1913, the Northern Pacific had its first superheated Mallets made for the operation of heavy, slow freight trains over the Cascades and in the Rocky Mountains. The ten locomotives came from ALCO in Schenectady and had the wheel arrangement 2-8-8-2. Four were coal-fired and destined for the Rocky Mountains, while the other six were oil-fired for the Seattle Division. A total of eleven more from ALCO Brooks followed in 1917 and 1920.

With cylinders measuring 26 and 40 inches in diameter, respectively, they developed more than 85,000 pounds of pulling power. However, these had such a high steam consumption that the highest power was already available at 10.6 mph. In operation, it had been shown that they could pull almost 30 percent more load as pilot compared to a class Z-1 locomotive while consuming the same amount of coal. In the cascades they pulled up to 2,400 short tons alone over most of the run, but were helped by another Mallet on the steeper sections with up to 2.2 percent.