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In 1887 Anatole Mallet conceived the idea of a articulated locomotive , able to negotiate tight curved track but with both sections of the machine to be powered and that the swivel section could employ a low pressure steam cylinder to add to the overall power. This section swiveled and so the steam supply needed to swivel also, by using a compound system the used steam for the high pressure cylinder would not stress the swivel jointed steam supply pipe too much !.
Below is a modern example of Anatole’s patent. The High pressure cylinders (2) are fixed to the main frame of the locomotive, these exhaust steam at reduced pressure to supply the Low pressure cylinders (70-80 psi). The low pressure cylinders articulate just like a road truck ( semi) and are part of the BLUE section of the engine. We have two locomotives in one , the idea being that this machine does the work of two standard locomotives. = 2 x 2-8-2’s . The weakness being the jointed steam supply, this did present problems .. The “A” class Mallet of the N&W depicted is not a TRUE Mallet as most of them were SIMPLE, that is to say single pressure locomotives with all cylinders using boiler pressure ( 300 psi). The “Y” class were true Mallet engines having 39 inch LP cylinders and 25 inch HP cylinders see photo above. Huge lumps of engineering ? You bet !
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