The pivoting casting inside a coupler that rotates to engage the coupler of an adjacent car. Knuckles are the most frequently replaced coupler component because they bear the buff and draft forces of train handling. A broken knuckle will cause a train to separate and requires immediate repair.
The mechanical device at each end of a railcar that links cars together into a train. North American railroads use the AAR standard E or F knuckle coupler, which engages automatically when two cars are pushed together. Coupler failures are a leading cause of train separations and bad orders.
A railcar that has been removed from service because it has a mechanical defect, damage, or other condition that makes it unsafe or unsuitable to move loaded. Bad-ordered cars are set out at a repair facility until they are inspected and restored to serviceable condition. The term is used as both a noun and a verb in railroad operations.