HowTo/Make a Track Cutting
From TrainzOnline
(Also known as an "Undercut" in US and Canada)
Lay a section of track or road over the top of hill with a single pair of spline points at either end. Note that the track follows the contour of the hill.
Click on Advanced > Smooth Spline (S) and then Click on the road. This will level the terrain between the two spline points, thereby creating a cutting through the hill.
