Designed by the locomotive engineer and inventor Anatole Mallet (1837-1919), a Mallet type locomotive is a four cylinder compound articulated locomotive. Mallet locomotives have essentially two steam engines mounted under the same boiler. The rear engine is rigidly attached to the boiler, while the front engine is able to swing laterally around a pivot point located near the rear, high-pressure cylinders (see figure 1). This articulation allows the locomotives to negotiate curves that would not accommodate a large rigid-frame locomotive.
On a compound Mallet, live steam from the boiler enters the rear cylinders, and it is the exhaust from the rear engine that powers the forward low-pressure cylinders (see figure 2). To compensate for the lower temperature and pressure of the exhaust steam, the low-pressure cylinders are much larger in diameter that the rear set. This compounding system allows for savings on fuel and water, however the difficulties in setting valves to deal with the volume of low pressure steam, and the slow speeds of compound Mallets led to most American railroads abandoning compounds by the 1920s. Exceptions to this were such locomotives as Norfolk & Western's famous Y class 2-8-8-2s, which were still being built as compounds into the 1950s, and of course the logging Mallets, of which all but six were compounds.
While it is not technically correct to call them Mallets since they are not compounds as per the Mallet patent, simple articulateds like Weyerhaeuser #111 were identical in design concept to a regular Mallet except for the fact that live steam was delivered to both sets of cylinders at the same time (see figure 3). With simple articulateds the fuel economy benefits of a true Mallet were lost, however, the four Weyerhaeuser simple logging Mallets had higher tractive effort and a speed advantage over Weyerhaeuser's compound Mallets. Major visual differences between the two types were the identically sized cylinders and double stacks of the simple locomotives. Weyerhaeuser and Uintah were the only roads to experiment with simple logging 2-6-6-2s, although the Simpson Logging Company did consider purchasing one in the late 1920s.