A coupling (or a coupler) is a mechanism for connecting rolling stock 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 all rolling stock can be coupled together.
The different types of coupling do not always have formal or official names, which makes descriptions of the couplings in use on any railway system problematic.
The standard type of coupling on railways following the British tradition is the buffer and chain coupling used on the pioneering Liverpool and Manchester Railway of 1830. These couplings followed earlier tramway practice but were made more regular. The vehicles are coupled by hand using a hook and links with a turnbuckle-like device 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. In contrast, Janney couplers encourage violent encounters in order to engage the coupling fully. The earliest buffers were fixed extensions of the wagon frames, but later spring buffers were introduced.
Inefficient and slow, the European system is relatively unsafe because it requires manual coupling between vehicles, exposing workers to the risk of being crushed. With central couplers (except link and pin) it is not usually necessary to get between the cars for coupling or uncoupling. The safety issue was one of the main arguments for changing to central couplers. After more than 30 years, the change has still not been completed; indeed it has barely started.
This coupling type is the standard in European countries (except the former Soviet Union, where the SA-3 automatic coupler is used). Coupling is done by a worker, who must climb between the cars. First he turns a releasing screw (an aid with two opposite windings, and it does not uncouple the train itself) to the loose position, and then he can hang the chain on the hook. After hanging the chain on the towing hook the releasing screw must be turned to the tight position. When the coupler is uncoupled, it must be hung on the idle hook to prevent damage to itself or the brake pipes. Only shunting is permitted with a dangling chain. Disconnected brake pipes must be hung on hooks. (The picture shows two coupled cars, with a single brake pipe.)
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. That means there are no run-in forces on the coupler. The other benefit compared with automatic couplers is that its lesser slack causes smaller forces on curves; there is a lower probability of a broken coupler in a curve than with automatic couplers. The disadvantage is the smaller mass of the freight that can be hauled by that coupler (maximum 3000 tonnes).
Early rolling stock was often fitted with a pair of auxiliary chains as a backup if the main coupling failed. This made sense before the fitting of continuous fail-safe braking systems.
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.
A peculiarly British institution was the "loose-coupled" freight train. This used three-link chain couplings with no means of drawing the wagons together: since such trains were not fitted with an automatic through-train braking system there were no pipes to connect between the vehicles. The couplings in the train were kept taut by the last vehicle of the train being a heavily ballasted guards van with its brakes set slightly on. This helped prevent snapped couplings. Such trains travelled at low speeds and were phased out in the 1970s.
An improvement on this is the "Instanter" coupling, in which 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 without the need to wind a screw. The closeness of the coupling allows the use of inter-vehicle pipes for train brakes. It also has the advantage that it can be operated entirely from the side of the wagons using a shunter's pole and is therefore safer when shunting work is under way. These couplings are still prevalent in UK freight trains today.
On some narrow-gauge lines in Europe a simplified version is used, consisting of a single central buffer with a chain underneath. It's called Balancierhebelkupplung. The chain usually contains a screw-adjustable link to allow close coupling. On sharp curves, a single centre buffer is less likely to be subject to buffer locking problem, as described above.
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 and some freight cars of the Indian Railways are fitted with a 'transition coupler' that incorporates a screw coupling within a knuckle coupler: 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. See SAB WABCO C-AK 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 makes of carriages converted to use knuckle couplers.
Some Russian locomotives and wagons have buffers together with the central coupler. When coupling to Finnish equipment, a short chain with a block that fits in the central coupler is placed on the Russian side, backing up and compressing the buffers so that the chain can be laid on the hook. (That is also the common way of coupling locomotives to or from wagons, faster than unscrewing the link.)
British locomotive-hauled passenger carriages adopted a dual coupling system in the 1950s. They have retractable buffers and a central Buckeye automatic knuckle coupler that lowers to reveal a hook for a screw-type chain coupling. Inter-stock coupling was with the automatic coupler (with the buffers retracted), while connection to the locomotive was with the buffer-and-chain system with a screw coupler.
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 did not 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 not use the club, and risk injury.
The link and pin coupler proved unsatisfactory because:
An episode of the 1960s TV series Casey Jones was devoted to the problems of link and pin couplings.
Norwegian (or meat chopper) couplings consist of a central buffer with a mechanical hook that drops into a slot in the central buffer. The Norwegian is found only on narrow gauge railways , such as the Ffestiniog Railway and the Welsh Highland Railway, 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, and still applies to railways in New Zealand. 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. 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 Norwegian couplers as its standard, and converts Janney coupler to Norwegian as required. The slot in the "buffer beam" where the coupler protrudes appears to be about the same for both types of couplers. As a museum, it is appropriate to use the older type of coupling.
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.
The knuckle coupler or Janney coupler was invented by Eli H. Janney, who received a patent in 1873 (). 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.
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 coupling was chosen to be the American standard, there were 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.
The AAR coupler has stood the test of time since its invention, and has seen only minor changes:
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.
The 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 highly 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/3.2 km/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 Portland, Minneapolis, the Vancouver Skytrain, and the Scarborough RT in Toronto.
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 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) 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 coupling on a wagon, and present, for example, a meatchopper coupler to the next wagon. Such an adaptor might weigh 100 kg.
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.
Articulated sets of carriages or wagons share the intermediate bogies, and have no need for couplings in the intermediate positions.
Russia and Central Asia used buffer and chain couplings, albeit with possibly wider centres for the buffers, until conversion to automatic SA-3 couplers. The SA-3 coupler was invented in 1932. Some wagons were equipped with SA-3 couplers in the 1930s (they could be coupled with chain coupling), but all cars have received automatic couplers in 1957. See also: Intermat/Willison coupler, Image:Sa3.gif, Автосцепка СА-3 (in Russian)
Electronically Controlled Pneumatic brakes (ECP) need a method of connecting electrically adjacent wagons, both for power and for command signals, and this can be done by plugs and sockets, or by very short range radio signals.
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 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. Once the Kadee patents ran out, a number of other manufacturers began to manufacture similar (and compatible) couplers.
Recently, an exact-scale HO model of the AAR coupler has been designed and manufactured by Frank Sergent, of 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 British 00 scale (similar to H0 scale) models the 'tension lock' coupler is standard. This is similar in operation to the meatchopper type of coupling. 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.