One thing I've learned from living in Philly
...is that the moment you think you've seen everything, you discover something new in a place you've been a million times before. This site, now abandoned, was part of the Reading Viaduct, once a major transportation channel to and from the city. It cuts right through a portion of Center City directly over the street. You can see its underbelly from the street, but there are very few points of entry to gain access to the tracks above.