The Mallet Locomotive is a type of articulated locomotive, invented by a Swiss engineer named Anatole Mallet (and thus, the name is properly pronounced in the French manner, "Mallay").
The Mallet locomotive has two "engines"-- i.e. two pairs of cylinders, each pair rod-connected to a group of driving wheels. The axles of the rear set of drivers turn in the main frame of the locomotive, which is fixed to the boiler above it (at the rear end, to allow expansion of the hot boiler). The frame holding the front set of drivers is hinged to the main frame, allowing it to swing from side to side. The front end of the boiler rests on a sliding bearing on the swinging front frame.
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Mallet's original design was a compound locomotive, in which the steam is used twice, first in a set of high-pressure cylinders, then in a set of low-pressure cylinders. This confers certain thermodynamic advantages, and also worked well with the Mallet design. Steam was fed from the steam dome down to the aft, high-pressure cylinders — the exhaust steam from those being fed forwards in a pipe with a swiveling joint — to the forward, low-pressure cylinders. The exhaust steam from the larger low-pressure cylinders is exhausted through a slit in the sliding bearing in the top of the swiveling truck and thus to the smokebox above, and the blastpipe (US: exhaust nozzle) and chimney (US: stack). The difference in size between the high and low pressure cylinders can be seen in the picture of the 2-6-6-2 Mallet.
Unlike the case of the rigidly-framed locomotive, the Mallet design is easier to build as a compound, since steam and exhaust pipes are needed for both pairs of cylinders when it is built as a simple. When built as a compound, the only flexible pipes that are needed are the ones that deliver low-pressure steam from the rear cylinders to the front.
Mallet's original patent specifies compound expansion, but after his death in 1919 many locomotives (particularly in the United States) were articulated Mallet style without using compounding (for instance the Union Pacific Big Boy). When fleets of such locomotives appeared in the middle 1920s the trade press naturally called them "Simple Mallets" — i.e., simple locomotives articulated like Mallets — but eventually the notion spread that this was somehow incorrect. The term "Mallet" continued to be widely used for simples as well as compounds, but enthusiasts who like to consider themselves purists insist "simple Mallet" is an oxymoron.[1]
Mallet's original design was intended to allow a medium-size locomotive to better negotiate the tight curves of a narrow gauge railway, but the design was particularly attractive to railroads in the United States because it permitted locomotives to be built to sizes impossible with a single, rigid frame. It was introduced to the USA by the Baltimore & Ohio Railroad in 1904.[2] The 4-8-8-4 "Big Boy" used by Union Pacific Railroad and the 2-6-6-6 Allegheny of the Chesapeake and Ohio Railway were the heaviest steam locomotives in the world; the 2-8-8-2 Y6b class of the Norfolk and Western Railway was considered the ultimate development of the Mallet compound locomotive. These engines were built through World War II, with the last Y6b being constructed in 1952. However, outside North America, the Mallet type had generally been superseded by the Garratt locomotive by the mid 1920s, although in the Dutch East Indies, now the Republic of Indonesia, several types and sizes remained in use into the 1980s, and after Indonesian independence in 1949 the Indonesian State railways DKA ordered a series of 0-4-4-2's, basically an updated version of the earlier Dutch design, in 1962, destined for the old Atjeh (now Aceh) tramway. Constructed by Nippon Sharyo in Japan, these are the only Mallets built in Asia. In contrast to the rest of the Indonesian railways it has a gauge of 750 mm (2'6"), as to 1067 mm (3"6") for the rest of the Archipelago. Smaller Mallets were used by plantations and other industries, all of the 0-4-4-0 type. These ran mostly on 600 mm (1' 11.5") and 700 mm (2' 3.5") gauge networks.
Several Mallets have been preserved, some even in operational condition. A number of the Union Pacific "Big Boys" have escaped the scrapper, including one overlooking the US city of Omaha, where UP is based. One of the early Indonesian Mallets (formally a Dutch colony) has been returned to the Netherlands and is now part of a setup in the Dutch National Railway Museum (Nederlands Spoorweg Museum, NSM), while another industrial type has been purchased and restored by the Statfold Barn Railway in the UK. This will see its first operations in Europe in 2011 and after initial trials on the owners railway, will be transferred to the Ffestiniog and Welsh Highland Railways as its power is better suited to that railway.