The Horse Shoe Trail runs from a junction along the Appalachian Trail (in St. Anthony's Wilderness) to Valley Forge, PA.  Unfortunately this trail seems to be disappearing. Hopefully we can get more hikers and equestrians out there keeping the trail open!  I have only hiked 20 miles of this trail, but hope that it stays open as a walking route from Harrisburg to Philadelphia.
Check out this trail's website:
http://www.hstrail.org/