by a 2-8-8-2. The AT&SF also had a number of compound 2-10-10-2's, assembled in their own shops from existing 2-10-2's using a kit, supplied by Baldwin, consisting of the front 10-wheel frame and a boiler extension. Although compounds had been considered obsolescent since the 1920s, C&O thought them appropriate, in the late 1940s, for low-speed coal-mine pickup runs converging on the classification yard at Russell, Kentucky. Only ten (of 25 originally ordered) were built before the order was cancelled, the last delivered in September 1949. The final loco, Chesapeake and Ohio 1309, is preserved on the Western Maryland Scenic Railroad. The 1309 was also the last steam locomotive that Baldwin built for the North American market. The last compound Mallets to remain in use on a major North American railroad were the N&W class Y6b 2-8-8-2 locomotives, retired in July 1959. Norfolk & Western 2156 is the sole surviving Y6a, preserved at the National Museum of Transportation in St. Louis. Simple expansion versions in the US By about 1920, the U.S. version of the Mallet as a huge slow-speed pusher had reached a plateau; the size of the low-pressure cylinders became a limiting factor even on the large loading gauge permitted in the U.S., and reciprocating masses posed serious dynamic problems above walking pace. Moreover, there were adhesion stability problems where the front engine tended to slip and then stall uncontrollably because of an imbalance of tractive effort and axle load, accentuated by the drawbar reaction, and inability of the intermediate steam receiver to accommodate the sudden pressure change. This was further worsened by dynamic instability of the front end in running. The Chesapeake and Ohio Railway introduced 25 simple (non-compound expansion) 2-8-8-2 locomotives in 1924 and 20 more in 1926. Although the simple-expansion concept diverged from Mallet's original patent, the locomotives were clearly a continuation of the concept and were still referred to as "Mallet" locomotives. As the front truck cylinders were now using boiler pressure steam, special arrangements were necessary to deliver it, through the truck pivot pin where only radial movement took place. These new locomotives took over service on a division; a single locomotive hauled in five hours. Mallet development culminated in 1941 with the 4-8-8-4 Big Boy type on the Union Pacific railroad. They weighed with a tender; at long (including the tender), they could only be turned on a few of the system's turntables.