railroad 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 tracks. In contrast to road transport, where the vehicles run on a pre ...

ing, slack action is the amount of free movement of one car before it transmits its motion to an adjoining coupled car. This free movement results from the fact that in railroad practice cars are loosely coupled, and the

coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...

is often combined with a shock-absorbing device, a "

draft gear A coupling (or a coupler) is a mechanism typically placed at each end of a railway vehicle that connects them together to form a train. A variety of coupler types have been developed over the course of railway history. Key issues in their desi ...

", which, under stress, substantially increases the free movement as the train is started or stopped. Loose coupling is necessary to enable the train to bend around curves and is an aid in starting heavy trains, since the application of the locomotive power to the train operates on each car in the train successively, and the power is thus utilized to start only one car at a time.

United Kingdom

The UK formerly used

three-link coupling Buffers and chain couplers (also known as "buffers and screw", "screw", "screwlink", and "English" couplers) are the de facto UIC standard railway stock coupling used in the EU and UK, and on some surviving former colonial railways, such as in ...

s, which allowed a large amount of slack; these were soon replaced on passenger stock by

buffers and chain coupler Buffers and chain couplers (also known as "buffers and screw", "screw", "screwlink", and "English" couplers) are the de facto UIC standard railway stock coupling used in the EU and UK, and on some surviving former colonial railways, such as in ...

s where the couplings are held tight by buffers and shortened by a turnbuckle while in most other parts of the world automatic couplings, such as the

Janney coupler Janney couplers are a semi-automatic form of railway coupling that allow rail cars and locomotives to be securely linked together without rail workers having to get between the vehicles. They are also known as American, AAR, APT, ARA, MCB, knuck ...

and the

Scharfenberg coupler The Scharfenberg coupler (german: Scharfenbergkupplung, abbreviated ''Schaku'') is a commonly used type of fully automatic railway coupling. Designed in 1903 by Karl Scharfenberg in Königsberg, Germany (today Kaliningrad, Russia), the coupler ...

, were adopted from the late Nineteenth Century on. Three-link couplings are a rarity in modern use.

Gallery

File:Consett-tankwagon41 - buffer beam.jpg, Three-link coupling on an antique tank wagon Image:Railroad coupler.agr2.jpg, AAR Type E couplers on

flatcar A flatcar (US) (also flat car, or flatbed) is a piece of rolling stock that consists of an open, flat deck mounted on a pair of trucks (US) or bogies (UK), one at each end containing four or six wheels. Occasionally, flat cars designed to carry ...

s File:Carrage couplings on ABT Railway (3939380014).jpg, Screw-tensioned three-link coupling, shown attached but not yet tensioned; when tightened, the turnbuckle draws the buffers together, eliminating jarring and shocks when starting or slowing the train. The narrow buffers of the left-hand vehicle are sprung, the thicker buffers on the right contain a hydraulic damper. The sprung buffers allow for some train articulation even when the cars are drawn firmly together

References

Rail technologies {{rail-transport-stub

United Kingdom

Gallery

See also

References