Knuckle couplers are a type of coupling mechanism used primarily in rail transport to connect train cars. They consist of a hinged, knuckle-shaped design that allows for easy coupling and uncoupling of cars while maintaining a secure connection during movement. This design helps to accommodate the movement and sway of train cars as they travel along tracks. These couplers are commonly found on freight trains and passenger trains, providing a reliable way to link multiple cars together. The American Association of Railroads (AAR) has established standards for knuckle couplers, ensuring compatibility and safety across different rail systems.