Northern Pacific 4-6-6-4 No. 5107 belonged to the first group of this type ordered by the NP, the Z-6 class delivered in 1936 by the American Locomotive Company. These articulateds weighed 624,500 pounds minus tender, and produced 104,267 pounds of tractive effort. Their four cylinders measured 23x32 inches, they sustained 250 p.s.i. of boiler pressure, and their driving wheel diameter was 69 inches. With 5832 square feet of evaporative heating surface, they had 2114 square feet of superheating surface. Their 152 square feet of grate area made them rapid consumers of the low-BTU lignite or "brown" coal that came from the Northern Pacific's own local sources. No. 5107 was one of the last NP 4-6-6-4s to operate, being scrapped in 1958. Wayne Koch provided this builder's photo from his collection.