nothing wrong with this design itself but some details of his example are problematic. 6km down from the midway point between LA and SanFran is loaded with faults and I'm no geology expert either but this close to the ring of fire I bet rock in that vicinity would be pretty toasty. Sure, maybe not exactly magma but could easy be like 160F.

You could pump out all (or even some) of the air, which would not only reduce friction but also help buffer the heat problem in the deepest parts due to the insulating qualities of the vacuum (like a Thermos).

There'd still be some major engineering involved; ATW the deepest transit tunnel at present, the Seikan Tunnel in Japan, is only about 140 meters below the seabed and 240 meters below sea level.

On the other hand, there's a mine in South Africa that's nearly 4km deep, and people work down there for extended periods of time. The mine has several hundred km of tunnels, too.

Last I heard, Elon promised to dig into the Hyperloop after his 1st quarter earnings were announced. This seems like a fine time to toss around ideas for what it actually might be.

Interesting idea, but doesn't fit with "I think we could actually make it self-powering if you put solar panels on it" part

Why not? Don't get me wrong, there are tons of problems with this, but it is clearly going to need some external power. Why not solar panels?

My apologies, I was remarking that it didn't seem to fit with Elon's description of the Hyperloop but neglected to notice that you weren't trying to predict what his actual idea will be.

You could of course hook the system up to a power grid that could be getting power from solar panels, but Elon's description sounded like he intended for the panels to be part of the system, which would require it to be above ground

I see what you're saying. Related, he talked about the system possibly being able to store energy for when solar wasn't available. I'm very interested in how that would work.