Let's see if I can put this into words; most of the authoritative sources differ slightly in defining it because it's not a single, simple formula, but more of a locally-derived set of calculations. "Ruling grade" is frequently used in railfan circles and some published works to describe the steepest grade on a given piece of railroad. Ruling grade is actually the grade which a railroad uses to determine tonnage ratings, motive power needs, etc., between Point A and Point B. In some locations, ruling grade might well end up being the same as the maximum grade, but in many cases ruling grade can be less than the maximum grade. This is because ruling grade takes into account all forms of resistance between Point A and Point B, whether it's fluctuations in gradient or the number and severity of curves. Example: the NS profile for Saluda Mountain shows a short stretch of 4.8 percent about a mile below the summit, but with the many stretches of 4.1, 4.2, and 4.3 percent either side of there, the ruling grade for the mountain has been set at 4.70 percent. During a cab ride up Cajon in 1989, then-Santa Fe spokesman Mike Martin told me the North Track, which is generally understood to be 2.2 per cent, actually has a short stretch of almost 3 per cent between the Cajon X-overs and Sullivan's Curve. Almost 20 years later, the BNSF track chart covering the new third main track alignment indeed shows a less than quarter-mile stretch of 2.82 per cent. Not to be confused with the 3 percent on what was once called the South Track. Train momentum can normally overcome short blips like this or the one on Saluda, so they're often not heard of when people refer to the ruling grade.