Among the most important inventions of the 19th century were the air brake and the automatic coupler. Today most American rolling stock is equipped with air brakes, which operate automatically if a coupling between cars breaks or if a leak develops in the compressed-air system. In the 1870s the American inventor Eli Hamilton Janney patented a design for couplers with pivoted knuckles that would interlock automatically when two cars were pushed together and that could be disengaged by means of a lever extending to the side of the car.
In 1887 all car builders adopted as a standard a modified form of the Janney coupler. Because public resentment was aroused by the number of men crushed between cars when operating the old link-and-pin couplers by hand, a federal law was passed in 1897 requiring the installation of automatic couplers on all rolling stock. Enforcement was delayed by later legislation, but eventually this law became effective.
On American passenger cars the head of the buffer at each end of the car is a horizontal plate that slides over or under a corresponding plate on the next car to form a connecting platform. In improved types of draft gears connecting couplers with car sills or underframes, sliding-friction devices have superseded springs.
Early in the history of railroading, buffers on the ends of cars were introduced to minimize shocks when cars were bumped together. Modern designs make use of friction between two surfaces.
An important 20th-century advance in rolling-stock design was the introduction of roller bearings, which replaced sleeve bearings on car axles.