Members of the SP's AC-7 class of cab-forwards, represented here by No. 4173, came from Baldwin Locomotive Works in 1937 with a General Steel Castings one-piece engine bed for both sets of drivers. They mustered a boiler pressure of 250 p.s.i., with dimensions of both sets of cylinders being 24x32 inches. They weighed 639,700 pounds and produced 124,300 pounds of tractive effort. With a grate area of 139 square feet, they had an evaporative heating surface of 6505 square feet and a superheating surface of 2988 square feet (reduced to 2300 after World War II). This image of No. 4173 was acquired via eBay and is attributed to J. R. Quinn, but information about date and location is lacking.