Vertical clearance (clearance height)

Vertical clearance (or clearance height) is the vertical gap between the railhead and whatever sits above it — the underside of a bridge, a tunnel bore, or the upper deck of a multi-level layout. It is chronically underestimated: on top of the plain vehicle height come the sub-roadbed and ballast of the upper track, and with catenary a good deal more. Set too tight, an underpass is almost impossible to correct after building — especially in a helix, where every turn costs height and the radius is already tight.