Three main types of frame on steam locomotives may be distinguished:
Plate frames These used steel plates about thick. They were mainly used in Britain and continental Europe. On most locomotives, the frames would be situated within the driving wheels ("inside frames"), but some classes of an early steam locomotive and diesel
shunters were constructed with "outside frames". Some early designs were
double framed where the frame consisted of plates both inside and outside the driving wheels. Others were
sandwich frames where the frame was constructed of wood sandwiched between two metal plates.
Bar frames locomotive These are openwork girder structures built up from steel or iron bars which are usually thick, welded into a single load-bearing assembly. They were first used on the
Bury Bar Frame locomotive during the 1830s, and were widely used in nineteenth century American locomotives (including those exported to Australia and New Zealand; see
Vogel railways).
Cast steel beds Cast steel locomotive beds were developed in the latter years of steam locomotive design in the United States, from where they were also exported to Britain and Australia. == Articulated locomotives ==