Well, if this "Underground Philadelphia: Subways, Railways & Underground Architecture Walking Tour" doesn't sound like an enticement to one's own imagination...

"Discover the Philadelphia Plan which gave us two Art Deco masterpieces, the monumental 30th Street Station and Suburban Station, the first all-underground rail terminal. The tour starts at 30th Street Station and proceeds via trolley (token provided) to 19th St. From the Comcast tower lobby descend into an underground concourse network to The Gallery, before ending at Reading Terminal and the Marriott Hotel. Along the way, lament the loss of Broad St. Station and learn about the 1905-mid-50s trolley and subway lines, the Broad Street subway (1928), the Commuter Rail Tunnel (1984), and the former Reading Terminal, now part of the Pennsylvania Convention Center. (This tour requires climbing some stairs, though escalators and elevators are available at most points.) "

Meet at the Angel statue by the main concourse of the east entrance of 30th Street Station.

... then I honestly don't know what you're doing reading this website.

More info/Facebook invite here. Also happens on additional dates.