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A 'locomotive frame' is the structure that forms the backbone of the railway locomotive, giving it strength and supporting the superstructure elements such as a cab, boiler or bodywork. The vast majority of locomotives have had a frame structure of some kind. The frame may in turn be supported by axles directly attached to it, or it may be mounted on bogies (UK) / trucks (US), or a combination of the two. The bogies in turn will have frames of their own.

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Articulated locomotives

Articulated locomotives

An articulated locomotive with no fixed wheels (i.e. excluding the Mallet locomotive but including other articulated steam locomotives, as well as most diesel and electric locomotives) may have a separate frame beneath the superstructure, or the bodywork's internal structure may be load-bearing. Rarely is a true monocoque structure used.
Diesel and electric locomotives with a traditional, full-width body, known as cab units in North America, tend to have their strength in an internal structure. This style of construction is still popular elsewhere, but North American locomotives nowadays are overwhelmingly hood units—with a strong frame beneath the superstructure that carries all the load, and bodywork made of removable panels for easy maintenance. Fully enclosed locomotives are used in some limited applications, mostly for passenger trains. These tend to be cowl units, in which the body is not load-bearing.