Coupler is a vehicle component used to realize the connection between locomotive and vehicle, transmit traction and impact force, and keep a certain distance between vehicles. Couplers are divided into two types according to the opening mode: upper acting type and lower acting type. The lifting mechanism at the upper part of the hook head is called upward moving type (this type is mostly used for general freight cars); Push-down type (used by passenger cars) is opened by the push rod action at the lower part of the hook head. Couplers are divided into spiral couplers, close-fitting automatic couplers, automatic couplers and rotating couplers according to structural types. Spiral couplers were first used, but due to many shortcomings, they have been eliminated. High-speed railway vehicles mostly use close-fitting automatic couplers. In China, all heavy-haul unit trains on Daqin Railway adopt automatic couplers except rotary couplers.