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The term "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. Articulated locomotives are generally used either on lines with extreme curvature—logging, industrial, or mountain railways, for example—or to allow very large locomotives to run on railways with regular track curvature. Articulation is also applied to some electric locomotives, such as the Italian FS Class E656.
Articulated locomotives saw service in many nations, but were very popular on narrow-gauge railways in Europe and saw their greatest size developed in the United States, where the Union Pacific Big Boy 4-8-8-4s and the Allegheny H-8 2-6-6-6s were some of the largest steam locomotives ever built.
Many different 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 type (mostly built in the United Kingdom, popular throughout Europe, Africa and European colonies), and the various geared steam locomotive types, the latter largely used in logging, mining and industry. Most other types saw only limited success.
Articulated steam locomotive types
The major types of articulated locomotive are as follows:
- The Fairlie, with two powered trucks under a double boiler, or its Single Fairlie single-boiler derivative with one powered and one unpowered truck (known as a Mason Bogie in the United States)
- The Garratt, 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, 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, or simple, articulated steam engines had two sets of equally sized cylinders. When the steam was used in the firebox, the high pressure and low pressure came out the smokestack at once, resulting in higher tractive effort at the cost of more fuel burnt. Examples include the Challenger, EM-1, Cab Forwards, Big Boy, Allegheny.
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. This, however, resulted in lower tractive effort because of its low efficiency. Examples include the N&W Y6, 2-6-6-2 Type Engines, Virginian and Erie Triplexes, Old Maude, and even the N&W A Class.
There were various types of articulated geared steam locomotive, including:
- 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
There are several classes of articulated electric locomotives of generally two types:
- Three sections, where the middle part sits on the two outer parts (similar to the Garratt design), e.g.
- Two sections which share a central or Jacobs bogie, e.g.
Electric and diesel bogie locomotives have many construction aspects in common with Meyer type steam locomotives but are not seen as articulated.
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