Given that there are underground several tunnels within Pittsburgh that PennDOT pays a lot to maintain, it wouldn't be much of a stretch to believe one or two could be expanded to fit in a park. Thelowline.org does the job explaining the logistics and details with such a project. Pittsburgh isn't a congested as NYC, but the point of this thread was innovation. It's certainly a futuristic idea that has been considered in several cities in addition to NYC.