Crest/sag

Crest and sag are the two shapes of a vertical transition in the height profile: the crest where a grade turns into the level (or into a downgrade), the sag where a downgrade turns into a climb. Both are troublesome if the kink is too abrupt: over a crest, long stiff vehicles lift their wheels and couplers can part; in a sag, low-hanging parts — coupler, valance, pilot — bottom out. Prototype and model alike ease such transitions (vertical easement); as a model rule of thumb a level section sits between opposing grades.