Meaning of ruling grade

rul'ing grade"

Pronunciation: [key]
— Railroads. Railroads.
  1. the steepest grade on a given stretch of track, which determines the maximum tonnage that can be hauled in a train having a given horsepower at a stated minimum speed.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.