The Spokane, Portland & Seattle Railway was jointly owned by the Great Northern and the Northern Pacific Railroads. Most of its steam motive power came to it through purchases of second hand locomotives from its parents. Of the 121 steam locomotives owned by the SP&S throughout its existence only 30 were purchased new and none were of its own design. The eight "Challengers" it owned were new and came as add-ons to Northern Pacific orders.

In 1937 the SP&S needed locomotives and its owners added six 4-6-6-4s to an order placed by the Northern Pacific and in 1944 two more "Challengers" were added to another NP order.

The six "Challengers" received in 1937 were built by the American Locomotive Company and were identical to NP Class Z-6 except they burned oil. On the SP&S they were also designated Class Z-6 and were assigned road numbers 900 through 905. These locomotives had (4) 23 x 32 cylinders, 69" drivers, a 250 psi boiler pressure, exerted 104,500 lbs of tractive effort and weighed 621,000 pounds. Numbers 900 and 901 were sold to the Great Northern Railroad shortly after they arrived in 1937. Number 900 (GN number 4000) returned in 1946 and number 901 (GN number 4001) returned in 1950, both being deemed surplus on the parent railroad.

The two ALCO-built 4-6-6-4s received in 1944 were also oil burners and were designated Class Z-8 and given road numbers 910 and 911. These locomotives had (4) 23 x 32 cylinders, 70" drivers, a 260 psi boiler pressure, a tractive effort of 107,000 lbs and weighed 639,000 pounds.

The eight SP&S "Challengers" were scrapped in 1960.