Ruling gradient

The term ruling grade is usually used as a synonym for "steepest climb" between two points on a railroad, but if the steepest climb is, say, a quarter-mile of 1% preceded and followed by 0.5% the "ruling grade" can only be defined arbitrarily. More simply, the steepest grade to be climbed dictates how powerful the locomotive must be (or how light the train) in order to complete the run without assistance. Even if 99% of the line could be run with a light (and cheap) locomotive, if at some point on the line there is a steeper gradient that a light engine would be unable to climb, this gradient "rules" that a more powerful (and expensive) locomotive must be used, in spite of it being far too powerful for the rest of the line. This is why special "helper engines" are often stationed near steep grades on otherwise mild tracks, because it is cheaper than running a too-powerful locomotive over the entire track mileage just in order to make the grade, especially when multiple trains run over the line each day.