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.