The costs of tunneling are like FAR more expensive than most think. Breaking, excavating, and supporting rock is slow, time and cost heavy, and precarious work. While this is an interesting concept, unless there are serious advances in rock boring techniques (personal opinion: there are none coming) this will never approach fruition. I would suggest anyone interested in further research look into the "Big Dig" of Boston and the staggering costs and challenges it faced.
Good luck, Elon. It'll be another moonshot company if you can pull it off.
Good luck, Elon. It'll be another moonshot company if you can pull it off.