Caves, tunnels and other underground features?

[Apologies if this is covered somewhere here already, I haven't really figured out a good way to search individual forums here yet..]

I'm implementing an above and below ground train/rail system in my world. It's one big loop that circles the map and there are parts above and parts bel\ow. Above ground is no problem, but I'm not sure where to even begin with doing something underground. Is it possible to have unity terrains stack making multiple levels? What about normals? Like I said, not even sure where the best direction to go with this.. but I figure it's been brought up at some point already.

TY!

 

For underground features you basically want to use a mesh based solution where you model out the interior sections in the software of your choice. You could use multiple terrains, but it'd be extremely cumbersome.