Ridership I like is low on the Philly subway network, because the communities it serves are not the communities downtown workers live in anymore. So they don't capture a large amount of commuters. Low ridership also can be an issue of public safety. The Philly subway has a very bad reputation safety wise. And while fights happen everywhere, the following fight shows what most people probably think riding the Septa subway is like on a daily basis.
http://ca.youtube.com/watch?v=Ygvdw3...eature=related
Second before expanding the subway, I would increase service on the current subway system, and on the suburban rail lines.
You can increase service on some of the suburban rail lines to like every 10 minutes and provide frequent rapid transit service to many points in the city like Manyunk, etc.