Philadelphia has a large unused underground system that was part of SEPTA.
“The underground Center City Concourse system has provided pedestrians with a way to avoid walking at street level for almost 90 years. Originally built by the Philadelphia Transit Company, these tunnels were sold to the City in 1968 when the newly formed Southeastern Pennsylvania Transportation Authority couldn’t afford to purchase the vast subterranean system from the PTC. The abandoned stretch of tunnels…”
https://hiddencityphila.org/2016/12/underground-concourse-revival-sees-light-of-day/