Mallet locomotive

The Mallet Locomotive is a type of articulated locomotive
Articulated locomotive
Articulated locomotive usually means a steam locomotive with one or more engine units which can move independent of the main frame. This is done to allow a longer locomotive to negotiate tighter curves...

, invented by a Swiss engineer named Anatole Mallet
Anatole Mallet
Jules T. Anatole Mallet was a Swiss mechanical engineer, who was the inventor of the first successful compound system for a railway steam locomotive, patented in 1874....

 (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
A boiler is a closed vessel in which water or other fluid is heated. The heated or vaporized fluid exits the boiler for use in various processes or heating applications.-Materials:...

 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.

Compound expansion

Mallet's original design was a compound locomotive
Compound locomotive
A compound engine unit is a type of steam engine where steam is expanded in two or more stages.A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure cylinder, then having given up heat and losing pressure, it exhausts directly into one or more larger...

, 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
Steam dome
A Steam dome is a vessel fitted to the top of the boiler of a steam locomotive. It contains the opening to the main steam pipe and its purpose is to allow this opening to be kept well above the water level in the boiler...

 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
A smokebox is one of the major basic parts of a Steam locomotive exhaust system. Smoke and hot gases pass from the firebox through tubes where they pass heat to the surrounding water in the boiler. The smoke then enters the smokebox, and is exhausted to the atmosphere through the chimney .To assist...

 above, and the blastpipe
The blastpipe is part of the exhaust system of a steam locomotive that discharges exhaust steam from the cylinders into the smokebox beneath the chimney in order to increase the draught through the fire.- History :...

 (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.

Simple expansion

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
Union Pacific Big Boy
Big Boy was the name of the Union Pacific Railroad's 4000-class 4-8-8-4 articulated steam locomotives, built between 1941 and 1944 by American Locomotive Company...

). 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
An oxymoron is a figure of speech that combines contradictory terms...



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. The 4-8-8-4
In the Whyte notation for the classification of steam locomotives by wheel arrangement, a 4-8-8-4 is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel trailing truck.Other equivalent classifications are:...

 "Big Boy
Union Pacific Big Boy
Big Boy was the name of the Union Pacific Railroad's 4000-class 4-8-8-4 articulated steam locomotives, built between 1941 and 1944 by American Locomotive Company...

" used by Union Pacific Railroad
Union Pacific Railroad
The Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest railroad network in the United States. James R. Young is president, CEO and Chairman....

 and the 2-6-6-6
The 2-6-6-6 is an articulated locomotive type with 2 leading wheels, two sets of six driving wheels and six trailing wheels. Only two classes of the 2-6-6-6 type were built. One was the "Allegheny" class, built by the Lima Locomotive Works. The name comes from the locomotive's first service with...

 Allegheny of the Chesapeake and Ohio Railway
Chesapeake and Ohio Railway
The Chesapeake and Ohio Railway was a Class I railroad formed in 1869 in Virginia from several smaller Virginia railroads begun in the 19th century. Led by industrialist Collis P...

 were the heaviest steam locomotives in the world; the 2-8-8-2
.A 2-8-8-2, in the Whyte notation for describing steam locomotive wheel arrangements, is an articulated locomotive with a two-wheel leading truck, two sets of eight driving wheels, and a two-wheel trailing truck. The equivalent UIC classification is, refined to Mallet locomotives, D1...

 Y6b class
Class (locomotive)
Class refers to a group of locomotives built to a common design for a single railroad. Often members of a particular class had detail variations between individual examples, and these could lead to subclasses. Sometimes technical alterations move a locomotive from one class to another...

 of the Norfolk and Western Railway
Norfolk and Western Railway
The Norfolk and Western Railway , a US class I railroad, was formed by more than 200 railroad mergers between 1838 and 1982. It had headquarters in Roanoke, Virginia for most of its 150 year existence....

 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
A Garratt is a type of steam locomotive that is articulated in three parts. Its boiler is mounted on the centre frame, and two steam engines are mounted on separate frames, one on each end of the boiler. Articulation permits larger locomotives to negotiate curves and lighter rails that might...

 locomotive by the mid 1920s, although in the Dutch East Indies, now the Republic of Indonesia
Indonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...

, 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
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, these are the only Mallets built in Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...

. 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
Omaha may refer to:*Omaha , a Native American tribe that currently resides in the northeastern part of the U.S. state of Nebraska-Places:United States* Omaha, Nebraska* Omaha, Arkansas* Omaha, Georgia* Omaha, Illinois* Omaha, Texas...

, 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
Statfold Barn Railway
The Statfold Barn Railway is a mixed gauge railway of , and gauges, near Tamworth, Staffordshire. It is privately owned by Mr Graham Lee, chairman of LH Group Services Ltd – which in 2005 bought what remained of the Hunslet Engine Company....

 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.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.