American Street was originally the route by which the North Penn Railroad approached Philadelphia, so naturally industry developed along it, and when the Reading consolidated its passenger operations in the Reading Terminal, the sole track remaining in the middle of the street was used to switch the spurs there. It's much like Washington in that regard--except Washington, being a major street as well, was much, much busier.
http://maps.google.com/maps/ms?ie=UT...c886d51c7&z=13
One issue to note on that map is that 25th Street, while I'm pretty sure it's the widest street of its kind in South Philly, is covered by the elevated freight railroad I've thought for some time can be used for rapid transit as well. Would it be most beneficial to consolidate the light-rail and heavy-rail operations in that part of South Philly? Or to use a one-way pairing around 20th/21st?
Also, is it worth it to go out of the way to get to 30th Street, especially when you pass the 34th Street station? Or is 30th Street such a major transit hub, and the developments around there, so important that it is unquestionably necessary in normal operations to go there? And if it is, wouldn't installing flex-tracks along the unserviced segment of 33rd/34th still be a good idea? (Also in that regard: installing a track from 36th to 33rd/34th so that trains running the 40th Street Cutoff can access the loop?)