Elon should use the nuclear powered tunnel boring machines patented in the 60s to dril and melt rock-wall tunnels for the hyperloop.
I have a feeling residents would throw a complete shit-fit if someone proposed something like that. At best, you'd have clickbait headlines like "MUSK PROPOSES NUKING HIS WAY THROUGH YOUR BASEMENT".
It sounds like he's less concerned about disturbing the people above, as it's apparently not an issue at certain depths (I am not an engineer). Seems like the existing underground infrastructure might be a bigger problem.