Spokane Portland & Seattle 4-6-6-4 Class Z-6 900 was the first of six "Challengers" built by the American Locomotive Company in 1937. She had 69" drivers, four 23 x 32 inch cylinders, operated at 250 psi boiler pressure, and could produce 104,500 lbs of tractive effort. These locomotives were identical to NP Class Z-6 except they were oil burners. All of the SP&S "Challengers" were scrapped in 1960.