Two couplers join when their knuckles close around each other and their lock pins drop, and they separate when someone lifts a cut lever to open one. Knuckles are designed as the weak link, so a hard slack run-out breaks a knuckle rather than a car frame. Locomotives carry spare knuckles weighing around 80 pounds, and humping one back to a break, often in the dark, is a rite of passage for conductors.
We broke a knuckle 60 cars deep, I'm walking back with a new one on my shoulder, give me about forty minutes.
Conductors say they broke in two or pulled a knuckle; either way the brake pipe parted at the break and the train went into emergency.
Also heard as
- coupler knuckle
Related terms
Slack
The free play in couplers and draft gear that lets cars move a little toward or away from each other.
Pull the pin
To uncouple cars by lifting the coupler's pin with the cut lever; also, to retire from the railroad.
Glad hands
The coupling heads on the ends of air brake hoses, which lock together when two cars are joined.
Big hole
The emergency position of the automatic brake valve, which vents the brake pipe and stops the train as fast as possible.