Other than it was first proposed that way, and it might help in the event of nuclear war, why do this whole thing underground where the tunneling expenses are so high, the material that needs to be sealed is wildly variable (rock formations and fractures), and water pressure must be fought back while pulling a vacuum? Maintenance and construction are very problematic underground. And who wants to be underground on the quake-prone west coast, moving at 400 MPH?

Why not just run this thing on pylons above ground? That way you are piecing together sections of tubes designed to hold lower pressure? If you're going to do it, at least do it in a practical and common sense fashion.

But the expense is so phenomenal compared to standard rail construction - why do this at all?  Why do we need to be able to go from NY to LA in an hour? I would think our productivity is good enough, thanks, and we really don't need this at all.