Mac, Selector had some good advice about how to handle a grade where you need to pass over something that can be dropped below tabletop level. You can end up with two sets of 2% grades this way instead of a 4% grade. Believe me, although 4% doesn't sound like much more than 2%, it really is. It's the difference between a normal length train with two units having to pull the grade slower than on the flat and a train half the size with either four units up front or mid-train or end of train pushers. Even 2% is nothing to sneeze at but 4% starts to present real operating problems.
HO Scale Modeler