Coupling (railway)

2007 Schools Wikipedia Selection. Related subjects: Railway transport

Knuckle (AAR Type "E") couplers in use.
Knuckle (AAR Type "E") couplers in use.

A coupling (or a coupler) is a mechanism for connecting railway cars in a train. The design of these couplings is a standard almost as important as the railway gauge, since flexibility and convenience are maximised if the couplings can work together no matter what order they come in.

For a complete list of what railway uses which coupler see (JANE'S WORLD RAILWAYS)

Buffers and chain

Traditional buffer-and-chain coupler
Traditional buffer-and-chain coupler
Two cars coupled
Two cars coupled
Buffer and chain coupler with knuckle swung out of the way.
Buffer and chain coupler with knuckle swung out of the way.

The type of coupling established as standard on railways following the British tradition is the buffers and chain coupling found on the pioneering Liverpool and Manchester Railway of 1830. These couplings followed earlier tramway practice but were made more regular. The cars are coupled by hand using hook and link system with a turnbuckle-like device that draws the cars together. Cars have buffers, one at each corner on the ends, which are pulled together and compressed by the coupling device. This arrangement limits the slack in trains and lessens shocks. In contrast the Janney couplers (see below) encourage violent encounters in order to engage the coupling fully. The earliest buffers were fixed, being extensions of the frames of the wagons, but later on, spring buffers were introduced.

Although inefficient and slow, the European hand-coupled system is relatively safe for the rail workers because the buffers prevent them from being crushed between the cars.

The hooks and chain hold the carriages together, while the buffers keep the carriages from banging into each other so that no damage is caused. The buffers can be "dumb" or spring-loaded, or indeed a mixture.

Early rolling stock was often fitted with a pair of auxiliary chains, as a backup if the main coupling were to fail. This would have made sense before the fitting of continuous and failsafe braking systems, whether air- or vacuum-based.

On railways where rolling stock always pointed the same way, the chain might be mounted at one end only, as a small cost- and weight-saving method.

On German railways, one buffer is flatter than the other buffer which is slightly more rounded. This provides better contact between the buffers than would be the case if both buffers were slightly rounded.

Three link couplings

It is possible to make the buffer and chain coupler close up to a tight fit, without the need to wind a screw. The middle link of a three link chain is specially shaped so that when lying "prone" it provides enough slack to make coupling possible, but when this middle link is rotated 90 degrees, the length of the chain is effectively shortened, reducing the amount of slack. This variation is used only on freight wagons.

Problems with buffers and chain

The buffers and chain coupling system has a maximum load much less that the Janney coupling. Also, on sharp reverse curves, the buffers can get bufferlocked by somehow getting on the wrong side of the adjacent buffer. An accident at a Swiss station was caused by bufferlocked wagons in the 1980s.

Variation with gauge

The distance between the buffers tends to increase as the gauge increases, so that if wagons are changed from one gauge to another, the buffers will no longer match.

Dual coupling

It is possible to mount both buffers and chain and knuckle couplers on the same car, provided that one can swing out of the way.

Locomotives as well as some freight cars of the Indian Railways are fitted with a 'transition coupler' that incorporates a screw coupling within a knuckle coupler. However, the knuckle coupler remains in position and does not swing away when not in use. The screw coupling is mounted on a lug within the knuckle coupler. Most Indian freight cars use the knuckle coupler alone, without buffers, whereas passenger coaches almost exclusively use screw couplers and buffers. Exceptions are the new LHB coaches imported from Europe, and a few other rakes of carriages converted to use knuckle couplers.

Link and pin

The coupler pocket (just below the two lights) for a link and pin coupler on a 15 inch gauge speeder.
The coupler pocket (just below the two lights) for a link and pin coupler on a 15  inch gauge speeder.

The link and pin coupling was the original style of coupling used on American railways, surviving after conversion to Janney couplings on forestry railways. While simple in principle, the link and pin coupling suffered from a lack of standardisation regarding size and height of the links.

The link and pin coupler consisted of a tubelike body that received an oblong link. During coupling, a railworker had to stand between the cars as they came together and guide the link into the coupler pocket. Once the cars were joined, the employee inserted a pin into a hole a few inches from the end of the tube to hold the link in place. This procedure was exceptionally dangerous and many brakemen lost fingers or entire hands when they didn't get their hands out of the way of the coupler pockets; many more were killed as a result of being crushed between cars or dragged under cars that were coupled too quickly. Brakemen were issued with heavy clubs that could be used to hold the link in position, but many brakemen would forgo the club's use and risk injury.

The link and pin coupler ultimately proved unsatisfactory because:

  • It made a loose connection between the cars, with too much slack action.
  • There was no standard design, and train crews often spent hours trying to match pins and links while coupling cars.
  • The links and pins were often pilfered (due to their value as scrap metal), resulting in substantial replacement costs. John H. White suggests that the railroads considered this to be more important than the safety issue at the time (see reference below).
  • Crew members had to go between moving cars during coupling, and were frequently injured and sometimes killed.
  • Railroads wished to operate trains that were heavier than the link and pin system could cope with.

An episode of the 1960s TV series Casey Jones was devoted to the problems of link and pin couplings.


Meatchopper (also known as Norwegian) couplings consist of a central buffer with a mechanical hook that drops into a slot in the central buffer. The hook resembles a meat chopper, hence the name. The meatchopper is found only on narrow-gauge railways, where low speeds and reduced train loads allow a simpler system. On railway lines where rolling stock always points the same way, the mechanical hook may be provided only on one end of each wagon. This was the situation on the Lynton & Barnstaple (L&B), a narrow gauge line in Devon, England.

Meatchopper couplings are not particularly strong, and may be supplemented by auxiliary chains. The L&B originally used side chains in conjunction with Norwegian couplers, but these were found to be unnecessary with the slow speeds employed (10-15 miles per hour) and were removed within a year or so of the line opening in 1898.

The Pichi Richi Railway in South Australia uses meatchopper couplers as its standard, and converts Janney coupler to meatchopper as required. Fortunately, the slot in the "buffer beam" where the coupler sticks out appears to be about the same for both types of couplers. As a museum, it is appropriate to use the more old-fashioned type of coupling.

Not all meatchopper couplings are compatible with one another.

Buffer and chain

A simplified coupling found on some narrow-gauge lines in Europe consists of a single central buffer with a chain underneath. The chain usually contains a screw-adjustable link to allow close coupling.

Automatic couplers

There are a number of automatic train couplings, most of which are mutually incompatible.

  • AAR ( American Association of Railroads) coupler (also known as knuckle coupler and once known as Janney coupler, see below) used in Canada, the USA, Mexico, Japan, Australia, South Africa, Saudi Arabia, Cuba, Chile, Brazil, China and elsewhere.
    • Maximum tonnage as high as 30,000 t.
    • The AAR coupling comes in at least two sizes, "full size" and "three quarter" which are not compatible. Lighter weight railways, especially those of narrow gauge, with no need for interrunning, sometimes use smaller (3/4 or half size) versions of the AAR coupling. AAR (Janney) couplings are always righthanded.
    • The AAR couplers (or Janney couplings) are not necessarily mounted at the same height above rail, but within reason, this can be tolerated.
    • Uncoupling AAR couplers is done by lifting the coupling pin with a lever located at the corner of the railway car. This pin is locked when the coupler is under tension, so the usual uncoupling steps are to compress the coupling with a locomotive, lift and hold up the pin, then pull the cars apart. A side operated variant is called the "Sharon coupler".
    • Trains fitted with AAR couplers can have heavier loads than any other type of coupler. Thus the heaviest coal trains in New Zealand have AAR couplings even though the remainder of the fleet has the meatchopper kind. Also, long-distance freight trains in North America are quite commonly more than a mile (1.6 km) long, whereas this is unknown in Europe, where most freight trains still use the buffers and chain system.
    • See also "Janney Coupler" and "changes since 1873" below.
  • Russian SA3 coupler , also known as a " Willison Coupler with a Russian contour", (somewhat similar to Janney) used in Russia, former Soviet Union, Finland, Mongolia and Iraq. See also: Animation showing SA3 coupling (site only in Russian)
    • Maximum tonnage about 6,000 t.
  • European proposal coupler, (compatible with the Russian coupler) with automatic air, control and power connections. Implementation permanently delayed except for a few users.
  • Scharfenberg coupler used on electric trains - connects brake and controls. See Fully Automatic Couplings below.
    • Maximum tonnage about 1,000 t.
  • Note: There exist a number of other automatic train couplings similar to the Scharfenberg coupler, but not necessarely compatible with it. Older US transit operators continue to use these non-Janney electro-pneumatic coupler designs and have used them in service for decades.

Janney coupler

Later Master Car Builders Association coupler, now AAR ( American Association of Railroads) coupler, see also AAR coupler above.

Diagram of the top view of Janney's coupler design as published in his patent application in 1873.
Diagram of the top view of Janney's coupler design as published in his patent application in 1873.

Janney couplings are always righthanded.

The knuckle coupler (or Janney coupler) was invented by Eli H. Janney, who received a patent in 1873 ( U.S. Patent 138,405). It is also known as a "buckeye coupler", notably in the United Kingdom, where some rolling stock is fitted with it. Janney was a dry goods clerk and former Confederate Army officer from Alexandria, Virginia, who used his lunch hours to whittle from wood an alternative to the link and pin coupler.

In 1893, satisfied that an automatic coupler could meet the demands of commercial railroad operations and, at the same time, be manipulated safely, the United States Congress passed the Safety Appliance Act. Its success in promoting switchyard safety was stunning. Between 1877 and 1887, approximately 38% of all railworker accidents involved coupling. That percentage fell as the railroads began to replace link and pin couplers with automatic couplers. By 1902, only two years after the SAA's effective date, coupling accidents constituted only 4% of all employee accidents. In absolute numbers, coupler-related accidents dropped from nearly 11,000 in 1892 to just over 2,000 in 1902, even though the number of railroad employees steadily increased during that decade.

When the Janney coupling was chosen to be the American standard, there were an amazing 8000 patented alternatives to choose from.

The only significant defect of the AAR (Janney) design is that sometimes the drawheads need to be manually aligned.

Changes since 1873

The AAR coupler, originally called the Janney coupling, has stood the test of time since its invention, and has seen only minor changes:

  • Type "E" coupler, the original (plain) AAR coupler, derived from the Master Car Builders Association coupler.
  • Type "F" coupler, a "Tooth and socket" variation to prevent accidents, derailments and wrecks from uncoupling the couplers. The "tooth" on a loose coupler could puncture any tank car or other car carrying hazardous materials. Variations on the AAR type "F" coupler have been devised to provide extra protection, in case of derailments and train wrecks, to cars routinely carrying sensitive or hazardous loads. These variations of type "F" couplers, generally involving "shelves", remain fully compatible with standard AAR couplers, but tend to keep derailments and collisions from uncoupling the cars (thereby preventing the "tooth" of the couplers from piercing the ends of the cars).
  • The APTA (former AAR) standard type "H" coupler, a "Tooth and socket" variation used mostly, if not exclusively, on passenger cars. Types "F" and "H" couplers are also known as tightlock couplings. The Type "H" coupler is now under the supervision of the APTA ( American Public Transportation Association)
  • "pads" to reduce slack on passenger trains.
  • improvement to castings, etc. to increase maximum trailing load.
  • rotating-shaft couplers (type "F") introduced for use in rotary car dumpers
  • It is clear that the original Janney coupler is no longer compatible with the latest AAR couplers. A visual comparison between the original Janney contour and the current AAR contour (see the illustration of the "Diagram of the top view of Janney's coupler..." and the photograph of the "Knuckle couplers in use" elsewhere in this article) would stongly indicate that the original Janney contour and the current AAR contour (especially that of the knuckle itself) are no longer compatible. The current AAR contour dates back to the " Master Car Builders Association" coupler.
  • narrow gauge railways such as the Victorian Puffing Billy Railway use a miniature version of the AAR coupler.

Fully Automatic Couplings

Fully automatic couplings are those which make all connections between the rail vehicles (mechanical, air brake and electrical) without human intervention, in contrast to autocouplers which just handle the mechanical aspects. The majority of trains fitted with these types of couplers are multiple units, especially those used in mass transit operations.

There are a few designs of fully automatic couplers in use worldwide, including the Scharfenberg coupler, various knuckle hybrids (such as the Tightlock, used in the UK), the wedgelock coupling, Dellner couplings (similar to Scharfenberg couplers in appearance), and the BSI coupling.

Older US transit operators use non-Janney electro-pneumatic coupler designs that have been in service for decades.

Scharfenberg Coupler

Scharfenberg coupler
Scharfenberg coupler

The Scharfenberg coupler or Scharfenbergkupplung or "Schaku" is probably the most commonly used type of fully automatic couplings. Originating in Europe, it has gradually spread from transit trains to regular passenger service trains, although, outside of Europe, its use is generally restricted to mass transit systems. The Schaku coupler is superior in many ways to the AAR (Janney/Knuckle) coupler because it makes the electrical and also the pneumatic connections and disconnections automatic. However there is no standard for the placement of these electro-pneumatic connections. Some rail companies have them placed on the sides while others have them placed above the mechanical portion of the Schaku coupler. The main disadvantage to the Scharfenberg coupler is its low maximum tonnage, which makes it totally unsuitable for freight operations.

Small air cylinders, acting on the rotating heads of the coupler, ensure the Schaku coupler engagement, making it unnecessary to use shock to get a good coupling. Joining portions of a passenger train can be done at very low speed (less than 2 mph [or 3.2km/h] in the final approach), so that the passengers are not jostled about. Rail equipment manufacturers such as Bombardier offer the Schaku coupler as an option on their mass transit systems and their passenger cars and locomotives. In North America all the trains of the Montreal Metro are equipped with it, as are new light rail systems in Denver, Baltimore and New Jersey. It is also used on light rail vehicles in Minneapolis, the Vancouver Skytrain, and the Scarborough RT in Toronto.

Dual couplings and match wagons

If a wagon with one coupling system needs to be coupled to wagons with another coupling type there are four solutions. This may be needed when taking metro rolling stock from its manufacturer to the city where it is to be used:

  • hope that dual fitment of the couplings is possible.
  • use a match wagon(s) which have different couplings at either end.
  • use a coupling adaptor.
  • for emergency use only, use a rope. Also useful if a coupler is damaged or broken off.

Only some kinds of couplings coexist on the end of a wagon at the same time, because amongst other reasons they need to be at the same height. For example, in the Australian state of Victoria, engines had the AAR (Janney) coupler, with buffers, and the chain mounted on a lug cast into the AAR (Janney) coupler.

A match wagon or match truck (also known as a barrier vehicle / wagon in Britain) has different kinds of couplings at each end. If a pair of match wagons is used, a rake of wagons using coupling A can be inserted into a train otherwise using coupling B.

A coupling adaptor might couple to an AAR (Janney) coupling on a wagon, and present, for example, a meatchopper coupler to the next wagon. Such an adaptor might weigh 100 kg.

Sets of carriages

Automatic couplers like the Janney are safer in a collision because they help prevent the carriages telescoping. British Rail therefore decided to adopt a Janney variant for its passenger carriages, with the coupler able to swing out of the way for coupling to engines with the traditional buffer and chain system.

In New South Wales, sets of carriages were permanently coupled with a fixed bar, since the carriages were only ever disconnected at the workshops. Freight cars are sometimes coupled in pairs or triplets, using bar couplings in between.

Articulated sets of carriages or wagons share the intermediate bogies, and have no need for couplings in the intermediate positions.

Coupler conversion

From time to time, a railway decides that it needs to upgrade its coupling system from one that is proving unsatisfactory, to another that meets future requirements. This can be done gradually, which can create lots of problems with transitional incompatibilities, or overnight, which requires a lot of planning.


Japan converted its British-derived buffer and chain couplings to the American Janney coupling over a period of a few days in the early 1920s, after considerable preparation.


Australia, with its break of gauge, has always had different couplers on different systems, and has generally adopted gradual conversion.


While Europe has chosen an automatic coupler compatible with the Soviet one, except for some trial installation, no action has been taken to implement the conversion, due to cost.


Once Congress passed a law mandating conversion from the link and pin coupler to the Janney coupler, railroads in the United States had only a few years to implement the change. The railroads in North America form one unitary system, and uniformity of couplers is important for smooth interchange of rolling stock.

Latin America

Railways in Central and South America are fragmented by gauge, geography, and financial and technical heritage. While some systems have adopted the American Janney coupler, others retain the British buffer and hook (buffer and chain) coupler (see above).

Soviet Union

Russia and Central Asia used buffer and chain couplings, albeit with possibly wider centres for the buffers, until conversion to automatic SA3 couplers in the 1960s and 1970s. See also: Intermat/Willison coupler, Animation showing SA3 coupling (site only in Russian)

Middle East

While the middle east is mostly standard gauge, three different couplings appear to be in use (not counting Sharfenberg couplings on EMU trains). These are buffer and chain, American and Russian types. The proposed UAR standard appears to be the American type.


South of the Sahara, American Janney, and chopper couplings (not necessarily of compatible types) appear to account for most couplings. The preferred UAR standard is again the American Janney coupling.

Brake couplings

Just as railway couplings allow a train of wagons to be reordered, couplings are needed for any continuous braking systems.

See Brake (railway)


On model railroads, couplers vary according to scale, and have evolved over many years. Early model trains were coupled using various hook-and-loop arrangements, which were frequently asymmetrical, requiring all cars to be pointing in the same direction. In the larger scales, working scale or near-scale models of Janney couplers were quite common, but proved impractical in HO and smaller scales.

For many years, the "X2F" or "Horn-Hook" coupler was quite common in HO scale, as it could be produced as a single piece of molded plastic. Similarly, for many years, a "lift-hook" coupler developed by Arnold, a German manufacturer of N-scale model trains, was commonly used in that scale.

The chief competitor of both these couplers, more popular among serious modelers, was the Magne-Matic, a magnetically-released knuckle coupler developed by Keith and Dale Edwards, and manufactured by Kadee, a company they started. While they closely resemble miniature Janney couplers, they are somewhat different mechanically, with the knuckle pivoting from the centre of the coupler head, rather than from the side. A steel pin, designed to resemble an air brake hose, allows the couplers to be released magnetically; the design of the coupler head prevents this from happening unless the train is stopped or reversed with a mated pair of couplers directly over an uncoupling magnet. Once the Kadee patents ran out, a number of other manufacturers began to manufacture similar (and compatible) couplers.

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