This is off-topic but I am intrigued by your comment about the tunnels being empty during the day. That's a depressing thought. I don't know Philadelphia, is that because they don't lead to anywhere useful anymore or is it because people are afraid to use them? What a waste of infrastructure, either way.
I grew up in Montreal and lived for 25 in Toronto and both those cities have a fantastic underground system of subways, public areas, malls, etc. With our Canadian winters, it is maybe not surprising that people use these places a lot, but why don't they in Philly?