Janney coupler

Knuckle couplers are a semi-automatic form of railway coupling that allow rail cars and locomotives to be securely linked together without rail 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, MCB, Buckeye, tightlock (in the UK) or Centre Buffer Couplers.

There are many variations of knuckle coupler in use today, and even more from the past, some variants of knuckle include:

Janney: the American original, a rather finicky coupler; reportedly annoying to make open and close.

Castle: an improvement on the Janney design, and a step towards the modern knuckle.

Split Knuckle: a rarer type, which replaces the finger of the Knuckle (the little flap that actually links two knuckles together, one of the few moving parts) with one that has a cut out in the middle of it and a hole bored through it; this modification was designed to work with the older Link and Pin couplers in widespread use before, though could reasonably work with European style chain couplers too.

AAR: AAR, the modern American knuckle, they have several variants of their own; ranging from the standard knuckle, to long drawbar ones, to passenger models, to a type designed specifically for tank cars.