Looks like a great idea!

But, I'm curious how fresh air is pumped into the tunnels?

I'm guessing that there might be some sort of mechanism for this?

Maybe pressurized air pipes and/or ducts/fans at regular intervals, and/or some other form of ventilation system?

Observation: For future underground tunnel systems, if only electric vehicles are permitted to enter, then there might not be as hard a requirement for an apropriate ventilation system as there would be for fossil-fuel vehicles...