Ruling gradient

In railroading, the ruling grade is steepest grade on the rail line between two locations. Climbing the steepest part of the line dictates the minimum motive power needed, or how light the train must be, in order for the run to be made without assistance. While a low-powered (and inexpensive) locomotive can handle less-steep sections, which might be the majority of a run, the more powerful locomotive is needed for the steeper parts. Therefore, this steep section "rules" or controls the whole line, even though that requires more power than necessary for the other sections. 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 more powerful (and thus more costly) locomotive over the entire track mileage in order to make the grade, especially when multiple trains run over the line each day (to help justify the fixed daily cost of the helper operation).

In the 1953 edition of Railway Engineering William H. Hay says "The ruling grade may be defined as the maximum gradient over which a tonnage train can be hauled with one locomotive....The ruling grade does not necessarily have the maximum gradient on the division. Momentum grades, pusher grades, or those that must regularly be doubled by tonnage trains may be heavier." This means the "ruling grade" may change if the management chooses to operate the railroad differently.