A knuckle coupler is a type of coupling device used primarily in rail transport to connect train cars. It consists of a hinged mechanism that allows the couplers to lock together securely when two train cars are brought close. This design enables smooth and efficient coupling and uncoupling of cars, which is essential for train operations. The knuckle coupler is commonly associated with the American Association of Railroads (AAR) standard, which has become the industry norm in the United States. Its robust design enhances safety and reliability, making it a preferred choice over older coupling systems like the link and pin coupler.