Terrain cut holes/terrain mask

Request a feature to cut a hole in terrain or place a transparent terrain polygon that can see through the terrain.
My specific use case is tunnels; I want to make train tunnels because I like trains and my customers say they want a train simulator. That said, I could see a lot of use cases for airports, as many airports have complex underground terminals/parkings/train stations etc that may be difficult to depict with terraforming.