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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.
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.
Steam locomotive types
These are the major types of articulated locomotive:
- 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 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. They usually consisted of two smokestacks.
Examples: Big Boy, Challenger, Allegheny, A Class, Y Class, etc.
Compound articulated steam engines like Anatole's mallet, consist of two sets of unequally sized cylinders. The smaller cylinder near the cab contained high pressured steam and then it was passed into a larger cylinder set at the front of the engine in low pressure before exhausting through the smokestack. This, however, resulted in lower tractive effort because of its low efficiency. They usually consisted of one smokestack.
Examples: A Class, Y Class, "Old Maude", Z Class, Erie Triplex, Virginian Triplex, etc. Note: A and Y Class N&W steam could be simple or compound while Z Class N&W were only compound type articulates.
There were various types of articulated geared steam locomotive, including:
- du Bousquet locomotive
- Engerth locomotive
- Goelsdorf locomotive
- Golwé locomotive
- Hagans locomotive, such as the Prussian T 13 (Hagans variant)
- Klein-Linder locomotive
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)
- Two sections which share a central or Jacobs bogie
Electric and diesel bogie locomotives have many construction aspects in common with Meyer type steam locomotives but are not seen as articulated.
- Wiener, Lionel, Articulated Locomotives, 1930, reprinted 1970 by Kalmbach Publishing Company as ISBN 0-89024-019-1