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 mountainous railways, for example—or to allow very large locomotives to run on railways with standard track curvature.
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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.
These are the major types of articulated locomotive:
There were various types of articulated geared steam locomotive, including:
There are several classes of articulated electric locomotives of generally two types:
Two-unit locomotives, e.g. Bo-Bo+Bo-Bo locomotives are not considered to be articulated but are permanently coupled together.
Electric and diesel bogie locomotives have many construction aspects in common with Meyer type steam locomotives but are not seen as articulated.