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 Northern Pacific. 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 two-axle trailing bogie necessary to be able to carry the load.

21 members of the class Z-6 were delivered by ALCO-Schenectady in 1936 and 1937. The first four 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 driver diameter of 69 inches, they could also haul heavy passenger trains if necessary.

In 1941, six heavier locomotives followed with slightly different boiler dimensions, drivers one inch larger and a boiler pressure increased by 10 psi, which formed the class Z-7. 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 weight.

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 line 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.