In 1937 the American Locomotive Company delivered six class Z-6 4-6-6-4s to the Spokane, Portland & Seattle. They were an add-on to a 1936 order placed by part-owner Northern Pacific, and were virtually identical to the NP's Z-6 class except for burning oil instead of coal. They had a locomotive weight of 621,000 pounds and developed 104,267 pounds of tractive effort. With 69-inch drivers, they sustained 250 p.s.i. of boiler pressure and their four cylinders measured 23x32 inches. Their grate area totaled 152 square feet, and they had 5832 square feet of evaporative heating surface and 2114 square feet of superheating surface. No. 900, first of the class, posed for this ALCo builder's photo acquired from an eBay seller. All SP&S 4-6-6-4s were sold to the scrapper in 1960.