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 Bos…
Elon has stated that he believes it is possible to reduce tunnel creation speeds by 5 to 10 times, and that this is what they will work on doing: https://youtu.be/FPzpugrbwBE?t=120
He's going to involve the government? :|