An articulated locomotive is a steam locomotive with one or more engine units which can move relative to 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 for very large locomotives to run on railways with more standard track curvature.

The largest steam locomotives ever built, such as the Union Pacific Big Boy 4-8-8-4s, were articulated locomotives. They saw service in most nations, but were very popular on the narrow gauge railways of Europe and saw their greatest size developed in the United States.

Many different schemes for articulation were developed over the years. Of these, the Mallet locomotive and its simple-expansion derivative was the most popular, followed by the Garratt type (mostly built in the United Kingdom and the various geared steam locomotive types, the latter largely used in logging, mining and industrial uses. Most other types saw only limited success.

Types of articulated locomotive

These are the major types of articulated locomotive; there were large numbers of other designs.

• The Fairlie, with two powered trucks under a twin 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.
• Multiple geared steam locomotive types.