Meaning of center pin

cen'ter pin"

Pronunciation: [key]
— Railroads. Railroads.
  1. a cylindrical device passing through the center plates of both the body bolster and the truck bolster, permitting the truck to swivel beneath the car body while the stress is taken by the center plate.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.