From the Boring Company:
How can we reduce the cost of tunneling?
First, reduce the tunnel diameter. To build a one-lane tunnel, the tunnel diameter must be approximately 28 feet. By placing vehicles on a stabilized electric skate, the diameter can be reduced to less than 14 feet. Reducing the diameter in half reduces tunneling costs by 3-4 times. Secondly, increase the speed of the Tunnel Boring Machine (TBM). TBMs are super slow. A snail is effectively 14 times faster than a soft-soil TBM. Our goal is to defeat the snail in a race. Ways to increase TBM speed:
Increase TBM power. The machine’s power output can be tripled (while coupled with the appropriate upgrades in cooling systems).
Go electric. Current tunnel operations often include diesel locomotives. These can be replaced by electric vehicles.
https://www.boringcompany.com/faq/This actually makes sense and doesn't propose using the Hyperloop in the tunnel. Instead it would use an electric skate with the car sitting on top of the electric skate. People would definitely pay to use this to save time in LA traffic.
The website also says that being in a tunnel is one of the safest places to be during an earthquake. I'm not sure if that makes sense.