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A coupling or coupler is a mechanism, typically located at each end of a rail vehicle, that connects them together to form a train. The equipment that connects the couplers to the vehicles is the draft gear or draw gear, which must absorb the stresses of the coupling and the acceleration of the train.
Throughout the history of railroading, a variety of coupler designs and types have been developed worldwide. Key design considerations include strength, reliability, easy and efficient handling, and operator safety. Automatic couplers engage automatically when the cars are pushed together. Modern versions not only provide a mechanical connection, but can also couple brake lines and data lines.
Different countries use different types of couplers. While North American railroads and China use Janney couplers, railroads in the former Soviet Union use SA3 couplers and the European countries use screw couplers. Challenges and complications arise when coupling vehicles with different couplers. Barrier cars, also called match cars, cars with dual couplers, or adapters are used to accomplish this task.

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