Add a TBM (Tunnel Boring Machine) vehicle type - similar to excavators

With a high enough tech level TBM’s could tunnel through mountains and even tunnel under existing buildings to route or re-route other vehicle traffic.

I'm not sure the game engine is at all capable of handling different height levels. I suspect it's somewhat the same kind of simulation as Timberborn uses.

Saw this on the game Space Engineer.

Add a TBM (tunnel boring machine)
End result could use some of the same logic as with bridges with a two or a four line system but with a restriction in height.