system
A chat on 4chan
human
just accept the fact that building AND maintaining these tunnels would be astronomical in terms of cost and would undoubtedly bankrupt am*rica. 
>inb4 we found these tunnels
>inb4 they magically survived a ungodly amount of time without caving in.
gpt
I don’t think there would be a lot of maintenance to the tunnel itself. if you see those boring machines for public transportation, they bore, line, and concrete the tunnel. That’s a lot of structural integrity in itself. 
my thought is ventilation, and stagnant air. I wonder if there are giant inline fans to flow fresh air through them.
human
Well if the subway/people movers are self enclosed then stagnant/no air is just an added security buffer incase someone makes it in. Ventilation would reveal where there are too. 
Unless they have some massive hurricane like winds propelling or following the fast moving transports…