A Many British-built steam locomotives used a distinctive center-locking system, but some also had retaining clips around the smokebox door. The center-locking system was more complex to manufacture, but offered an airtight solution, preventing leaks that reduce the locomotive’s efficiency. In contrast, the U.S.-style clip system was simpler to operate, but more labor intensive to construct. So it was a trade off of simplicity in construction, but more ongoing maintenance in the British system, compared with more labor intensive construction with less operating efficiency using clips.
To remedy this, the British used a system of buffers between cars and locomotives to prevent the cars from crashing together. This gives such equipment a
distinctive look.
In the U.S., however, Eli Janney designed the first knuckle-type coupler and filed for a patent in 1873. That design forms the basis of most automatic couplers in use for freight around the world today.
The Federal Safety Appliance Act of 1893 forced the industry’s conversion to automatic couplers, which U.S. law required by 1898. This rule led to rapid widespread use and an 80 percent reduction in accidents involving couplers at the time.
The standard American knuckle coupler, known today as the AAR coupler after the Association of American Railroads trade group, is widely used around the world in the Americas, Africa, India, China, and Australia. Trains handling passengers and hazardous commodities use its variations.
Specialized automatic couplers are now used for heavy freight in Europe, and all modern multiple-unit passenger trains use some type of automatic coupler. These days such trains are often permanently coupled within the train-set using fixed steel tube connections, which are bolted together. This provides a more secure connection with reduced slack action, at the expense of operational flexibility. – Keith Fender
