The term ruling grade is usually used as a synonym for "steepest climb" between two points on a railroad. More simply, the steepest grade to be climbed dictates how powerful the motive power (or how light the train) must be in order for the run to be made without assistance. Even if 99% of the line could be run with a low-powered (and inexpensive) locomotive, if at some point on the line there is a steeper gradient than such train would be able to climb, this gradient "rules" that a more powerful locomotive must be used, in spite of it being far too powerful for the rest of the line. This is why special "helper engines" (also dubbed "Bankers") are often stationed near steep grades on otherwise mild tracks. It is cheaper than running a too-powerful locomotive over the entire track mileage j