Essentially how the Mallet worked was that a rigidly mounted engine
nearest the cab produced high-pressure steam that was then pumped to a
forward engine which was “hinged” and free to swivel (thus an
“articulated” frame) so the locomotive could much more easily negotiate
curves and less-than perfectly maintained track. A very economical
means of using steam the Mallet Type, coupled with both engines being
roughly center mounted over each set of driving axles, was able to
produce very high horsepower and adhesion with two sets of cylinders
(and because of the low pressure steam in the front engine its cylinders
were substantially larger than the rear cylinders).
Eventually, however, many of the “true” Mallets fell out of favor with railroads. While they were excellent at producing high horsepower and tractive effort (when production ended around 1,300 had been produced), the Mallet’s low gearing to accomplish such did not allow the locomotives to travel much faster than about 25 mph and thus, coupled with the coming of the diesel age, most were retired by the 1950s. Additionally, most railroads found the compound expansion design too complicated to operate in service. Instead, many found the simple expansion much easier to maintain while providing sufficient tractive effort and adhesion.
The two major railroads perhaps best remembered for their use of true Mallets in regular service included the Norfolk & Western, which operated a large fleet of 2-8-8-2s (listed as Class Y) as well as the Chesapeake & Ohio that owned a roster of 2-6-6-2s. To learn more about the history and operation of the compound steam locomotive please click here. Today, a handful of Mallet Types of different wheel arrangements have been preserved and two are operational; 2-6-6-2T #110 operating on the Black Hills Central Railroad based in Hills City, South Dakota and 2-6-6-2T #4 at the Niles Canyon Railway in Sunol, California.
Of note, while technically incorrect most refer to the term Mallet
interchangeably with articulated when describing large steam
locomotives. The term articulated in relation to steam designs simply
means any wheel arrangement that utilizes at least two sets of driving
with at least one set (usually the front) having the ability to swivel
to more easily negotiate curves. In any event, over the years this
really has become a non-issue although it is important in regards to the
historical context of steamers, their development through the years,
and what exactly each term meant in describing them.
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