A coupling (or a coupler) is a mechanism for connecting rolling stock in a train. The design of the coupler is standard, and is almost as important as the track gauge, since flexibility and convenience are maximised if all rolling stock can be coupled together.
The equipment that connects the couplings to the rolling stock is known as the draft gear.
The standard type of coupling on railways following the British tradition is the buffer and chain coupling used on the pioneering Planet class locomotive of the Liverpool and Manchester Railway of 1830. These couplings followed earlier practice but were made more regular. The vehicles are coupled by hand using a hook and links with a turnbuckle that draws the vehicles together. In Britain, this is called a screw coupling. Vehicles 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. The earliest buffers were fixed extensions of the wagon frames, but later spring buffers were introduced.
This coupling is still the standard in European countries (except the former Soviet Union, where the SA-3 automatic coupler is used).
The link-and-pin coupling was the original style of coupling used on North American railways. After most railroads converted to semi-automatic , the link-and-pin survived on forestry railways. While simple in principle, the system suffered from a lack of standardisation regarding size and height of the links, and the size and height of the pockets.
The link-and-pin coupler consisted of a tube-like body that received an oblong link. During coupling, a rail worker 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 did not get them out of the way of the coupler pockets in time. 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 not use the club, and risked injury.
The link-and-pin coupler proved unsatisfactory because:
An episode of the 1958 TV series Casey Jones was devoted to the problems of link-and-pin couplings.
To avoid the safety issues, Karl Albert, then director at the Krefeld Tramway, developed during the 1920s the Albert coupler, a key and slot coupler with two pins. Cars to be coupled were pushed together, both couplings moving to the same side. One pin was inserted, then the cars were pulled to straighten the coupling and the other pin inserted. This operation required less exact shunting. Due the single piece design, only minimal slack was possible. The system became quite popular with tram systems and narrow gauge lines.
During the 1960s most cities replaced them by automatic couplers. But even in modern cars, Albert couplers get installed as emergency couplers for towing a faulty car.
Norwegian (or meat chopper) couplings consist of a central buffer with a mechanical hook that drops into a slot in the central buffer. Setesdalsbanen - Many pictures of the Norwegian (chopper) coupler There may also be a U-shaped securing latch on the opposite buffer which is fastened over the top of the hook to secure it. The Norwegian is found only on of , or less, such as the Isle of Man Railway, Western Australian Government Railways, the Ffestiniog Railway and the Welsh Highland Railway, Tanzania, where low speeds and reduced train loads allow a simpler system. The Norwegian coupler allows sharper curves than the buffer-and-chain, which is an advantage on those railways.
On railway lines where rolling stock always points the same way, the mechanical hook may be provided only on one end of each wagon. Similarly, the hand brake handles may also be on one side of the wagons only.
Norwegian couplings are not particularly strong, and may be supplemented by auxiliary chains.
Not all Norwegian couplings are compatible with one another as they vary in height, width, and may or may not be limited to one hook at a time.
Eli H. Janney, who received a patent in 1873 (). Eli Janney — The Janney Coupler It is also known as a "buckeye coupler", notably in the United Kingdom, where some rolling stock (mostly for passenger trains) 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. The term Buckeye comes from the nickname of the US state of Ohio, the "Buckeye State" and the Ohio Brass Company which originally marketed the coupling. aec Ohio Brass company
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. 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 coupler was chosen to be the North American standard, there were 8,000 patented alternatives to choose from. The only significant disadvantage of using the Janney design is that sometimes the drawheads need to be manually aligned. Many AAR coupler designs exist to accommodate requirements of various car designs, but all are required to have certain dimensions in common which allow for one design to couple to any other.AAR Manual of Standards and Recommended Practices, Section S, Part III:Coupler and Yoke Details, Issue 06/2007
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, (similar to Scharfenberg couplers in appearance), BSI coupling (Bergische Stahl Industrie now Faiveley Transport) and the Schaku-Tomlinson Tightlock coupling.
There are a number of other automatic train couplings similar to the Scharfenberg coupler, but not necessarily compatible with it. Older US transit operators continue to use these non-Janney electro-pneumatic coupler designs and have used them for decades.
The WABCO N-type is sometimes referred to as the pin and cup coupler or spear coupler.
The Scharfenberg coupler Scharfenberg coupler (Scharfenbergkupplung or Schaku) is probably the most commonly used type of fully automatic coupling. Designed in 1903 by Karl Scharfenberg in Königsberg, Germany (today Kaliningrad, Russia), it has gradually spread from transit trains to regular passenger service trains, although outside 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 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 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 Portland, Minneapolis, the Vancouver Skytrain, and the Scarborough RT in Toronto. It also equips all the dedicated rolling stock used for the shuttle services in the Channel Tunnel.
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 state of Victoria, engines had the AAR coupler, with buffers, and the chain mounted on a lug cast into the AAR coupler.
A match wagon or match truck (also known as a "barrier vehicle" / wagon in Britain and "transition car" in North America) 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 or compromise coupler might couple to an AAR coupling on a wagon, and present, for example, a meatchopper coupler or rapid transit coupler to the next wagon. Such an adaptor might weigh .
In New South Wales, sets of carriages were permanently coupled with a fixed bar, since the carriages were disconnected only at the workshops. Freight cars are sometimes coupled in pairs or triplets, using bar couplings in between.
In the case of the draw gear behind the hooks, if any, absorbs the tension while the side buffers absorb the compression.
Some couplers may not even have a draw gear.
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 moulded plastic. Similarly, for many years, a "lift-hook" coupler known as the Rapido and 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 modellers, 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 center 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. An earlier, mechanically tripped version of the design had a straight pin extending down from the knuckle itself, which engaged a diamond-shaped mechanical "ramp" between the rails, which had to be raised above rail height when uncoupling was desired.
Once the Kadee patents ran out, a number of other manufacturers began to manufacture similar (and compatible) magnetic knuckle couplers.
Recently, an exact-scale HO model of the AAR coupler has been designed and manufactured by Frank Sergent. Sergent Engineering This design uses a tiny stainless steel ball to lock the knuckle closed. Uncoupling is achieved by holding a magnetic wand over the coupler pair to draw the balls out of the locking pockets.
In O scale, an exact-scale working miniature version of the "Alliance" coupler was manufactured from the 1980s by GAGO models in Australia. Since 2002 it has been marketed by the Waratah Model Railway Company. Waratah Model Railway Company European modellers tend to use scale hook and chain couplings.
In British 00 scale (similar to H0 scale) models the 'tension lock' coupler developed by Tri-ang is standard. This is similar in operation to the meatchopper type of coupling. Remote uncoupling is possible by using a sprung ramp between the rails. The design of the hooks is such that the couplings will not uncouple when under tension (instead depressing the ramp). When the train is pushed over the ramp, it will lift the coupling hooks as the train passes over. By halting the train over the ramp, it is split at this point. While it works well, it is often seen as ugly and obtrusive (although smaller designs are available, these are not always fully compatible with other models) and many British modellers prefer to retrofit either Kadee types or working hook and chain couplings.
A recent development is an interchangeable coupling which plugs into a standardised socket, known as NEM 362 and which can be easily unplugged as required. This allows the modeller to easily standardise on whatever coupling is desired, without individual manufacturers needing to change their coupling type.