Although modeled basically after Lima Locomotive Works' Boston & Albany 2-8-4 design, the MoPac's class BK-63 Berkshires were noticeably different in appearance with their outside-journal pilot truck, Walschaerts valve gear, Worthington type BL feedwater heater, Delta cast steel trailing truck, and two sand domes. Their driver diameter was 63 inches, their cylinder dimensions 28x30 inches, and their boiler pressure 240 p.s.i. The locomotive weight was 412,000 pounds and they developed 76,160 pounds of tractive effort. Their grate area reached 100 square feet, and they had 5116 square feet of evaporative heating surface and 2121 square feet of superheating surface. R. J. Foster snapped this image of No. 1921 at St. Louis in 1935; the photo was part of the Roy F. Blackburn collection and was acquired through eBay.