Frames in position, Pacific construction at Doncaster works (CJ Allen, Steel Highway, 1928)

A locomotive frame is the structure that forms the backbone of the

railway Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in Track (rail transport), tracks. In contrast to road transport, where the ...

locomotive, giving it strength and supporting the superstructure elements such as a cab,

boiler A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, centr ...

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 A bogie ( ) (in some senses called a truck in North American English) is a chassis or framework that carries a wheelset, attached to a vehicle—a modular subassembly of wheels and axles. Bogies take various forms in various modes of tran ...

( UK) / trucks ( US), or a combination of the two. The bogies in turn will have frames of their own.

Types of frame

250px, Preserved GWR 9017 showing outside frames Three main types of frame on steam locomotives may be distinguished:, p 255.

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 A switcher locomotive (American English), shunter locomotive (British English), or shifter locomotive ( Pennsylvania Railroad terminology) is a locomotive used for maneuvering railway vehicles over short distances. Switchers do not usually mov ...

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

250px, Bar frames of a WAGR U class 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 The Bury Bar Frame locomotive was an early type of steam locomotive, developed at the Liverpool works of Edward Bury and Company, later named Bury, Curtis, and Kennedy in 1842. By the 1830s, the railway locomotive had evolved into three basic ...

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 Steel casting is a specialized form of casting involving various types of steel cast to either final/net or near-net shape. Steel castings are used when iron castings cannot deliver enough strength or shock resistance.Oberg, p. 1332 Examples ...

locomotive beds were a development of the final years of steam locomotive design in the United States. They were also exported to Britain and Australia from the US.

Articulated locomotives

articulated locomotive An articulated locomotive is a steam locomotive (rarely, an electric locomotive) with one or more engine units that can move independent of the main frame. Articulation allows the operation of locomotives that would otherwise be too large to neg ...

with no fixed wheels (i.e. excluding the

Mallet locomotive The Mallet locomotive is a type of articulated steam railway locomotive, invented by the Swiss engineer Anatole Mallet (1837–1919). The front of the locomotive articulated on a bogie. The compound steam system fed steam at boiler pressur ...

but including other articulated

steam locomotive A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, wood) to heat water in the loco ...

s, as well as most diesel and

electric locomotive An electric locomotive is a locomotive powered by electricity from overhead lines, a third rail or on-board energy storage such as a battery or a supercapacitor. Locomotives with on-board fuelled prime movers, such as diesel engines or ga ...

s) may have a separate frame beneath the superstructure, or the bodywork's internal structure may be load-bearing. Rarely is a true

monocoque Monocoque ( ), also called structural skin, is a structural system in which loads are supported by an object's external skin, in a manner similar to an egg shell. The word ''monocoque'' is a French term for "single shell". First used for boats, ...

structure used. Diesel and electric locomotives with a traditional, full-width body, known as

cab unit In North American railroad terminology, a cab unit is a railroad " locomotive" with its own cab and controls. "Carbody unit" is a related term, which may be either a cabless booster unit controlled from a linked cab unit, or a cab unit that co ...

s 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 unit A hood unit, in North American railroad terminology, is a body style for diesel and electric locomotives where the body is less than full-width for most of its length and walkways are on the outside. In contrast, a cab unit has a full-width ...

s—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 train A passenger train is a train used to transport people along a railroad line. These trains may consist of unpowered passenger railroad cars (also known as coaches or carriages) hauled by one or more locomotives, or may be self-propelled; self p ...

s. These tend to be

cowl unit A cowl unit is a body style of diesel locomotive. The terminology is a North American one, though similar locomotives exist elsewhere. A cowl unit is one with full-width enclosing bodywork, similar to the cab unit style of earlier locomotives, b ...

s, in which the body is not load-bearing.

References

External links

* Frame, Locomotive {{Loco-stub