Ruling grades are those which limit the weight or length of a train of cars which may be hauled by one engine. They are frequently, although not necessarily, the steepest grades on the road. It is sometimes possible by a mere change in the method of operating the trains to very greatly reduce what must technically be considered the ruling grade of the road. When one or two grades on the road are considerably higher than all other grades, it is possible to use assistant engines on those grades, and thereby very greatly increase the weight or number of cars in a through-train load over the entire line. The weight of train will then be limited by the next lower grade, which may be a grade which offers but little more than one-half the total resistance of the grade which is operated by pusher-engines. This enables the train-load to be practically doubled. Such grades, which are called pusher grades, will be considered in a succeeding chapter.

When a grade is very short and it is never necessary to stop a train while on the grade, it is frequently possible to load up an engine with a far greater number of cars than could be run up an indefinite grade of that length. This has already been discussed in the chapter on Momentum Grades. These are the most common exceptions to the general statement that the ruling grade is usually the maximum grade of the road.

The selection of the general route of the road usually determines more or less definitely the ruling grade of the road, except as that may be modified by "development." The rate of ruling grade should bear some relation to the general character of the road which is to be built. A second-class or third-class road, which at its best will never be anything more than a branch line, is not justified in spending much money to reduce a ruling grade. On the other hand, a great trunk line is thoroughly justified in spending enormous sums in the construction of tunnels, deep cuts, high embankments or viaducts, in order to reduce the rate of the ruling grade. In this chapter we will endeavor to determine the financial relation between the lowest permissible grade and the money which may profitably be spent to secure it.