Simple Expansion Versions in The US
By about 1920 the US version of the Mallet as a huge slow speed pusher had reached a plateau; the size of the lp cylinders became a limiting factor even on the large load gauge permitted in the US, 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 & Ohio road 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 conveniently 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 113 mile division, covering it in 5 hours with 9,500 tons and a single locomotive.
Further development culminated in 1941 with the 4-8-8-4 Big Boy type on the Union Pacific RR. They weighed 760,000 to 772,000 lb with a 434,000 lb tender; at 133 feet long they could be turned on the system's turntables. They were designed for a top speed of 70 mph and could develop 3,000 drawbar horsepower at 10 mph.
Read more about this topic: Mallet Locomotive
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