This has been discussed in earlier threads some time ago although I don't have time to search back right now.
There are two sections of the Arch Street subway located east of Broad. They are not connnected to each other or any other part of the subway system and accessible through manholes from the surface.
The Henry Ave bridge at Wissahickon Creek was built with provision for subway.
Under the parking garage that once existed at the Sears on the Blvd was the shell for station. It no longer exists as the garage was demolished along with the Sears.
The thread mentioned IIRC had links to pictures of these at
www.phillyhistory.org. Could search through that website for them.
The book "Underground Philadelphia" has some discussion of these as well.