You can build track that goes underground, and it’s not too difficult to do. The key is to prepare the land ahead of time, because tracks, like footpaths, can only lead underground through a vertical land face that is at least two levels high. This rule applies to both the path section tunnelling into the ground and the one emerging out of the ground.
When your track approaches a vertical face like this, simply continue building as if the wall were not in the way; tunnelling take place automatically.
Once you begin building underground, the landscape switches into the Underground View. (If you need to get into this view later, use the Underground View option on the View Options menu–the eye button on the Toolbar.) Except for the sections where the track enters and exits the subterranean world, you build underground sections of track in the same way as you would any other.