Railway coupling 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. Today, most (if not all) EMUs including high-speed Shinkansen trains, and some DMUs use the Shaku-Thomlinson type coupling system, while locomotive-hauled trains use the Janney coupling and Tightlock coupling system.
Australia, with its breaks of gauge, has always had different couplers on different systems, and has generally adopted gradual conversion. Conversion to the Janney coupling is now virtually complete. Commonwealth Railways started with Janney couplings on its 1,435 mm (4 ft 8 1⁄2 in) standard gauge Trans-Australian line, and some railways, like the former Victorian Railways and the Queensland Railways, used dual couplers. Older couplers remain on Heritage railways.
The European network has traditionally been formed of many independent national railway networks with buffer and chain used near universally to allow the interchange of rolling stock. The European Union Technical Specifications for Interoperability (TSIs) for high-speed passenger rolling stock mandate the use of Scharfenberg Type 10-compatible couplings. The Type 10 includes "horns" to aid coupling on curves and include a function to provide standardised automatic air-brake connections; the coupling horn is often visible poking out at the front of the nose of high-speed trains.
For European freight, the TSIs mandate buffer and chain couplings at specified heights. The European system links to the former Soviet Russian-gauge network, where SA3 automatic couplers are used. Some research has been undertaken to choose an automatic freight coupler compatible with the Soviet one, but owing to widescale replacement cost, no action has been taken to implement the conversion, except for some trial installations. In many heavy-haul applications, such as for coal and iron ore, either US AAR-type couplers or Soviet SA-3 couplers are used. Conversion is made harder to justify because the existing buffer and chain coupling is almost universal.
Meanwhile, drawgear of new rolling stock is being built at a height suitable for conversion. The proposed European freight coupling is compatible with the SA3 coupler but adds integrated air and electrical connections. This standard would need to be revised to allow for the unforeseen development of electronically controlled pneumatic brakes.
Once Congress passed the Safety Appliance Act in 1893, 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, except for mass transit, form one unitary system, and uniformity of couplers is important for smooth interchange of rolling stock.
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 and successor states
Russian Empire and later Soviet Union used buffer and chain couplings, albeit with possibly wider centres for the buffers, until conversion to automatic SA3 couplers. The SA3 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 received automatic couplers in 1957.
While the Middle East is mostly standard gauge, three different couplings appear to be in use (not counting Scharfenberg couplings on EMU trains). These are buffer-and-chain, American, and Russian types.
South of the Sahara, Janney (AAR) and chopper couplings (not necessarily of compatible types) appear to account for most couplings. The preferred and proposed UAR standard is the American Janney (AAR) coupling.