As a coal hauler the Norfolk & Western was the last North American railroad to acquire new steam locomotives, constructing them as late as 1954 in its Roanoke shops. The Y6b class of 2-8-8-2s, erected in 1948-1952, constituted the final development of the Mallet compound articulated locomotive in which the steam from the rear high-pressure cylinders is reused in the larger front cylinders. No. 2171, seen in a Roanoke buider's photo from the 1951 edition of World Railways, represents this last group. Their cylinders measured 25x32 inches (rear high-pressure) and 39x32 (front low-pressure), and they carried a boiler pressure of 300 p.s.i. Their driver diameter was 58 inches, and their grate area totaled 106 square feet. Their 5655 square feet of evaporative heating surface was augmented by 1775 square feet of superheating surface. The Y6b class weighed 582,900 pounds and mustered 126,838 pounds of tractive force. No examples of the Y6b class survive, but No. 2156 of class Y6a is owned by the Museum of Transportation in suburban St. Louis and currently on lease to the Virginia Museum of Transportation.