What is the suggested maximum grade for a model railroad?

Ideally the maximum grade should be less than 3%. In other words, for every 100 inches of horizontal track, the track can be raised or lowered 3 inches. Keep in mind that prototype railroads try to keep the steepness of their grades as low as possible, even to the extent of lengthening their mainlines considerably in order to avoid a steep grade - the less of a grade the train has to travel, the more a locomotive can haul up the grade.