Elon Musk has a curious relationship with public funding and his companies. I don’t claim to have any figures behind it, but solar panels (SolarCity), electric cars (Tesla) and space travel (SpaceX) are heavily subsidized by tax dollars. How else do you fund a radical technology company?
Which brings us to tunnels. They are notoriously time-intensive and cost-overrunning. Consider the https://en.wikipedia.org/wiki/Brenner_Base_Tunnel, 34 miles long, projected to take 20 years. Lots of very smart people have been building tunnels for a very long time, and just because you make it smaller, doesn’t mean you eliminate the unknowns.
Consider the place Musk wants to put the tunnels, LA. Los Angeles has active oil drilling.
Me thinks that such a geological feature would make any sort of tunnelling project impossible to estimate.
The rights of property owners also figure into this. Property rights, while not enforced by the courts, do extend into the earth. Mining companies must buy the right to dig on private land. Of course, there are many mechanisms, such as eminent domain, which industry can abuse to get what they want. How will Boring Company acquire the rights to dig beneath private and public land? More subsidies?
How is this going to generate a profit? From tolls and ticketing? Thus again, turning an unused public resource into private profits.
The ground also supports structures above, I don’t claim to know how deep these tunnels are proposed to be, but too low, and temperatures are going to be very high. Too high and you risk subsidence and foundation displacement.
So clearly, there are many, many challenges which must be overcome, which are not legislative or technological in nature. Not that anyone should fear challenge, but we must consider the potential alternative uses of this technology.
Consider that a more immediate application of SpaceX technology is not getting to Mars, but for military purposes. Consider that a side-effect of self-driving cars is ubiquitous monitoring of an individual's location. There is an incredible potential for the technology of publicly-funded Musk companies to be utilized against the public, all of which wrap incredible consequences in a pleasing veneer of do-gooding.
Musk is very concerned about the future of humanity, certainly not all humans, otherwise he’d be working on more humble pursuits, clean water and education perhaps. Another function of such tunnels could be to provide for human survival during a sustained climate event, such as an ice age. These of course, will be limited in space and provide for only a few.
You will be able to trade your Tesla for a ticket to ride out the apocalypse.
Personally, I’d rather die above ground then try to live beneath it.