[VivaLFuego]

As with Chicago, Philadelphia really isn't dense enough to support anything other than radial rail lines emanating from a dense employment core. Actually, few places in the world are dense enough to support cross-town, intracity, non-CBD rail lines - the G in New York is that system's lowest ridership line, and every other NY subway line is oriented to serve either Lower Manhattan, Midtown, or both.

That said, Philadelphia indeed has several fairly dense areas that lack any rail connectivity on either the rapid transit or suburban rail networks.