Every coupling has a few inches of free movement from the knuckle fit and the draft gear springs, and over a long train that adds up to dozens of feet. Slack running in, when cars bunch toward the locomotive, or running out, when they stretch away, can break knuckles, pull drawbars and derail light cars placed ahead of heavy ones. Handling slack smoothly with throttle, air and dynamics is the skill that separates a good engineer from a rough one.
Take the slack out easy before you notch up, there's a cushion car in the middle and she'll snap a knuckle if you jerk it.
Bunched and stretched describe which way the slack sits, and engineers start a stretched train very differently from a bunched one.
Also heard as
- slack action
Related terms
Knuckle
The hinged, hook-shaped part of a railcar coupler that closes and locks to join two cars.
Cushion
A car fitted with hydraulic cushioning units at its couplers, built to protect fragile lading.
Bail off
To release the locomotive's own brakes during an automatic application so that only the cars do the braking.
Dynamic braking
Retarding force made by turning a locomotive's traction motors into generators and burning the energy off as heat.