A bit OT, but heck it's Sunday...
A couple examples of starting down this path exist, at least with respect to moving goods to decongest surface streets...
The Alameda corridor is a grade separated mostly tunnel that moves containers from the LA Port up to the rail terminal East of downtown Los Angeles. A subsequent extension (Alameda Corridor East) continues this further East by grade separating the railroad tracks from surface streets.
Alameda Corridor - Wikipedia
Most port cargo is bound for the train yard in Victorville. There was once discussion of building a Maglev directly from the ports to Victorville. This would have had multiple benefits, including drastically reducing diesel truck traffic, cleaning up the air in the corridor, and getting the goods to Victorville much faster. A 100 mile network moving shipping containers at 90mph. I always thought that use of Maglev technology really made the most sense versus using it to move people point to point, since there is no other maglev network to connect to on either end. Haven't heard much about this recently, though they are talking about separate truck lanes and using electric trucks.
Ports considering maglev trains to cut smog
Elon's Boring tunnels are essentially mini versions of Maglev, just using battery powered vehicles and skipping the expensive superconducting "rails". And by skipping the rails, it allow for much easier interconnecting of any/all tunnels built, much greater flexibility. I don't know whether a large build out of Boring tunnels will end up reducing surface congestion. It will be interesting to see. And now it seems like UAM will also be in the running for moving people from the suburbs to various city center locations much faster too:
Urban Air Mobility (UAM)
The OP statement of moving goods versus people is well taken, much less safety system required. Not sure if say a shipping container would fit in a Boring tunnel or not. And you would need a pretty large integrated tunnel system to make it economical to start moving goods around, kind of a chicken and egg problem. The Maglev from the Ports to Victorville was a point to point solution that would work, so no additional connections would be required.
Now, if Elon can get tunneling cheap and fast enough, a cargo tunnel from the ports to Victorville would generate an insane amount of revenue, if the 90,000 trucks per day estimate is accurate. The fuel savings alone of switching from diesel to electric would obliterate truck traffic on surface streets. The boring company has to have looked at this possibility. If not, I'll happily accept 0.001% of the net profit for coming up with the idea.
RT