Locomotive frame

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.

Three main types of frame on steam locomotives may be distinguished:[1] These used steel plates about 1–2 in (25.4–50.8 mm) thick.

These are openwork girder structures built up from steel or iron bars which are usually 4–7 in (100–180 mm) thick, welded into a single load-bearing assembly.

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.