Knuckle couplers are a semi-automatic form of railway coupling that allow rail cars and locomotives to be securely linked together without workers having to get between the vehicles.

Originally known as Janney couplers (the original patent name) they are almost always referred to as knuckles in the US and Canada (regardless of their actual official model name, nowadays generally various AAR types in North America), but are also known as American, AAR, APT, ARA,