Norfolk & Western 2156 is a four-cylinder compound articulated class "Y6a" "Mallet" type steam locomotive with a 2-8-8-2 (Whyte notation) wheel arrangement. The Norfolk & Western Railway built it in 1942 at its Roanoke Shops in Roanoke, Virginia as a member of the N&W's Y6a class. It is the strongest-pulling extant steam locomotive in the world, although it is not operational. It was retired from regular rail service in July 1959 and is now owned by the National Museum of Transportation in St. Louis, Missouri. In 2014, the museum leased it for five years to the Virginia Museum of Transportation in Roanoke, Virginia. It was towed from St. Louis to Roanoke on May 10, 2015, and towed back to St. Louis on June 10, 2020. History Historic significance Norfolk & Western 2156 is the sole survivor of the railroad's Y5, Y6, Y6a and Y6b classes (in final form referred to as the "Improved Y5/Y6 class"). These locomotives developed 152,206 pounds of tractive effort when built, with later modifications bringing them closer to 170,000 pounds. By comparison, the Union Pacific Big Boy locomotives developed of tractive effort. The only successful steam locomotives that developed somewhat more tractive effort, the Virginian AE class 2-10-10-2s, pulled trains at about 8 mph (13 km/h), while the N&W Y6's regularly pulled trains 50 mph (80 km/h), and some anecdotal evidence exists that they pulled trains up to 63 mph (101 km/h). N&W 2156 is also one of the Y6a's that received a new firebox with an extended combustion chamber of the type used on the Y6b class, which increased drawbar horsepower from 4400 hp (3.3 MW) at 20 mph (32 km/h) to 5600 hp (4.2 MW) at 25 mph (40 km/h). Steam versus diesel tests, upgrades, and controversies Coal traffic was N&W's largest source of revenue, and it had arguably the most modern and efficient steam locomotives of any major U.S. railroad. Accordingly, N&W resisted conversion from coal-burning steam locomotives to oil-burning diesels longer than most. In 1952, N&W tested its A-class and Y6b-class locomotives against a four-unit Electro-Motive Division F7 diesel set. The tests indicated that fuel costs and similar items were roughly the same, and the test was considered a tie. However, diesels eventually won out for lower maintenance and other operational costs. Retrospective analyses of these tests have led to suggestions that diesel locomotive builder EMD and N&W cheated in the competition