A heavy-duty chain or cable assembly that loosely connects a railcar's body to its wheel trucks. Under normal conditions, the truck pivots freely on the center plate. If the car derails or is lifted violently, the safety chain prevents the heavy truck assembly from tearing completely away from the car body, reducing the severity of a wreck.
Heard on the job
The car knocker bad-ordered the flatcar because the safety chain on the trailing truck was snapped.