A tunnel can have any kind of transport running in it as you want. Cars, buses, and trains at the same time. A toll tunnel that is run full will be wildly profitable and would inspire others to build them. "The Boring Company" already has the tech, we just need to work out the regulatory system so that they can build across the multiple small cities in the LA basin. Below a certain depth you can't even tell the tunnel is there.

A controlled tunnel system would be easily driven by today's automatic cars. It pick you up and goes back home (no parking problem), runs on electricity from your sunny California rooftop solar array, and works as a peaker power source for the grid when you aren't using it. Bring me a future that is better than the past please.