Not as straightforward as the title indicates. How do you take an existing coaster that was designed partially underground and build it as it was designed? If you lower the land to accommodate the track, is there a way to rebuild or replace the land above it?

I don't have RCTC, but going off of my knowledge of the original games I'd assume if you mold the terrain to have the same features as the original coaster blueprint had it should be able to fit right in there. If that doesn't work you can always place it and terraform around the spot yourself of course.

I've done that with hills before, but not underground.

To fill up the land above the track I switch to build mode and delete that portion of the track. Then I raise the land at that spot and, back in build mode, restore the track section. Hope this helps.