An articulated locomotive is a

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, Fuel oil, oil or, rarely, Wood fuel, wood) to heat ...

(rarely, an

electric locomotive An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a Battery (electricity), battery or a supercapacitor. Locomotives with on-board fuelled prime mover (locomotive), ...

) with one or more engine units that can move independently of the main frame. Articulation allows the operation of locomotives that would otherwise be too large to negotiate a railroad's curves, whether mainlines or special lines with extreme curvature such as

logging Logging is the process of cutting, processing, and moving trees to a location for transport. It may include skidder, skidding, on-site processing, and loading of trees or trunk (botany), logs onto logging truck, trucksindustrial, or

mountain railway A mountain railway is a railway that operates in a mountainous region. It may operate through the mountains by following mountain valleys and tunneling beneath mountain passes, or it may climb a mountain to provide transport to and from the su ...

s. Articulated locomotives saw service in many nations, but were very popular on

narrow-gauge railways A narrow-gauge railway (narrow-gauge railroad in the US) is a railway with a track gauge (distance between the rails) narrower than . Most narrow-gauge railways are between and . Since narrow-gauge railways are usually built with Minimum railw ...

in Europe. The largest examples were developed in the United States, where the

Union Pacific Big Boy The Union Pacific Big Boy is a type of simple articulated locomotive, articulated 4-8-8-4 steam locomotive manufactured by the American Locomotive Company (ALCO) between 1941 and 1944 and operated by the Union Pacific Railroad in revenue service ...

4-8-8-4 A 4-8-8-4 in the Whyte notation for the classification of steam locomotives by wheel arrangement, is a locomotive with a four-wheel leading truck, two sets of eight driving wheels, and a four-wheel trailing truck. Only one model of locomotives ha ...

s and the Allegheny H-8 2-6-6-6s were some of the largest steam locomotives ever built, with Big Boy 4014 remaining as the largest, and last of its kind, to still operate. Many schemes for articulation were developed over the years. Of these, the Mallet locomotive and its simple-expansion derivative were the most popular, followed by the

Garratt A Garratt locomotive is a type of articulated steam locomotive invented by the engineer Herbert William Garratt that is articulated into three parts. Its boiler, firebox, and cab are mounted on a centre frame or "bridge". The two other ...

type (mostly built in the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Northwestern Europe, off the coast of European mainland, the continental mainland. It comprises England, Scotlan ...

, popular throughout

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

and European colonies), and the various

geared steam locomotive A geared steam locomotive is a type of steam locomotive which uses gearing, usually reduction gearing, in the drivetrain, as opposed to the common directly driven design. This gearing is part of the machinery within the locomotive and should not ...

types, the latter largely used in logging, mining and industry. Most other types saw only limited success. As distinct from articulated locomotives, a non-articulated locomotive is referred to as a ''straight'' or ''rigid'' locomotive.

Articulated steam locomotive types

Major types

The major types of articulated locomotive are: * The Fairlie, with two powered trucks under a double

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centra ...

, or its Single Fairlie single-boiler derivative with one powered and one unpowered truck (known as a Mason Bogie in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

). * The Garratt locomotive, with an engine unit at each end carrying coal and water supplies, and a boiler unit articulated between them. * The Mallet locomotive, with one fixed engine under the rear of the locomotive and a radially swinging engine unit in front. * The

Meyer locomotive A Meyer locomotive is a type of articulated locomotive that has two separate Bogie, bogies, upon which the boiler and firebox swivel. The design was never as popular as the Garratt locomotive, Garratt or Mallet locomotive, Mallet locomotives. It ca ...

, with two powered engine trucks under the locomotive (generally with the cylinders inward). Also, the Kitson-Meyer variant with the trucks spread apart to allow a deeper firebox between them.

Simple expansion

Simple expansion, or simple, articulated steam engines had two sets of equally sized cylinders. High-pressure steam was supplied to all cylinders and exhausted out of the stack once it had been used. The American simple-expansion articulated, thanks largely to the smaller mass of the forward cylinders when compared to the compound-expansion Mallets allowing for higher piston speed, were generally better suited for high speed than their compound cousins. Examples of the "simple mallet" design include the

s and Challengers, the Chesapeake and Ohio class H-8, the

2-8-8-4 A 2-8-8-4 steam locomotive, under the Whyte notation, has two leading wheels, two sets of eight driving wheels, and a four-wheel trailing truck. The type was generally named the ''Yellowstone'', a name given it by the first owner, the Northern Pa ...

"Yellowstones", the majority of Southern Pacific's

Cab-forward The term cab forward refers to various rail and road vehicle designs that place the driver's compartment substantially farther towards the front than is common practice. Rail locomotives In steam locomotive design, a cab-forward design will ...

s, and the Norfolk & Western A-class.

Compound expansion

Compound expansion, or compound, articulated steam engines like Anatole Mallet's original idea, consist of two sets of unequally sized cylinders. The smaller pair of cylinders near the cab was fed with high pressure steam directly from the boiler and then the steam was passed into a pair of low-pressure cylinders at the front, with larger diameter to offset the lower pressure, before exhausting through the smokestack. While the

thermal efficiency In thermodynamics, the thermal efficiency (\eta_) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, steam turbine, steam engine, boiler, furnace, refrigerator, ACs etc. For ...

was greatly improved through the compound use of steam in Mallet designs, the large low-pressure cylinders posed unique limitations, both in terms of loading gauge (the cylinders could only be as large as the track and track-side infrastructure allowed) and in terms of performance at speed. The large and consequently heavier pistons caused stability issues at higher speed, which generally limited compound expansion articulated locomotives to below 30 or 40 miles per hour. A notable exception to this was to be found in later iterations of Norfolk & Western Y-class 2-8-8-2s, which could and did often exceed 50 miles per hour in service as well as being one of the hardest-pulling steam locomotives ever built. The first Garratt locomotives constructed, the Tasmanian Government Railways K class were also compound locomotives, but were complicated as a result. All subsequent Garratts were simple engines only.

Geared types

There were various types of articulated

, including: * Climax locomotive * Heisler locomotive *

Shay locomotive The Shay locomotive is a geared steam locomotive that originated and was primarily used in North America. The locomotives were built to the patents of Ephraim Shay, who has been credited with the popularization of the concept of a ''geared st ...

* Willamette locomotive

Other types

* du Bousquet locomotive * Engerth locomotive * Gölsdorf axle * Golwé locomotive * Hagans locomotive, such as the Prussian T 13 (Hagans variant) * Klien-Lindner locomotive * Krauss-Helmholtz bogie * Luttermöller axle

Electric locomotives

There are several classes of articulated

s of generally two types: * Three sections, where the middle part sits on the two outer parts (similar to the Garratt design), e.g. ** Milwaukee Road class EP-2 ** Swiss crocodile locomotives, :de:SBB Ce 6/8 II and :de:SBB Ce 6/8 III ** DRG Class E 94, :de:DRB-Baureihe E 94 * Two sections which share a central or Jacobs bogie, e.g. **

FS Class E.636 The Ferrovie dello Stato, FS E.636 is a class of Italy, Italian articulated electric locomotives. They were introduced in the course of the 1940s until the 1960s, and have been decommissioned since 2006. They have been one of the most numerous It ...

Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...

, first built in 1940 ** HŽ series 1061, a derivative of E.636 operating in

Croatia Croatia, officially the Republic of Croatia, is a country in Central Europe, Central and Southeast Europe, on the coast of the Adriatic Sea. It borders Slovenia to the northwest, Hungary to the northeast, Serbia to the east, Bosnia and Herze ...

- also known as SŽ series 362 in

Slovenia Slovenia, officially the Republic of Slovenia, is a country in Central Europe. It borders Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the south and southeast, and a short (46.6 km) coastline within the Adriati ...

and as JŽ series 362 in

Yugoslavia , common_name = Yugoslavia , life_span = 1918–19921941–1945: World War II in Yugoslavia#Axis invasion and dismemberment of Yugoslavia, Axis occupation , p1 = Kingdom of SerbiaSerbia , flag_p ...

** FS Class E.646 in Italy, first built in 1958 ** FS Class E.656 operating in Italy since 1975 ** Rhaetian Railway Ge 6/6 II in

Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...

** New Zealand EW class, built by

English Electric The English Electric Company Limited (EE) was a British industrial manufacturer formed after World War I by amalgamating five businesses which, during the war, made munitions, armaments and aeroplanes. It initially specialised in industrial el ...

in 1952. The conventional electric and diesel locomotive dual bogie design uses the same general configuration as the Meyer design but is not considered to be articulated.

References

* Wiener, Lionel, ''Articulated Locomotives'', 1930, reprinted 1970 by Kalmbach Publishing Company as

External links

Articulated locomotives

Articulated steam bogie locomotives
{{Whyte types Articulated locomotives Steam locomotive types