Just in case, here is a map to get from Route S to the Liberty Bell
Cue Sheet here
The trail head is right before the route S crosses 422. Turn left into the parking lot, go downhill towards the highway, the trail is obvious.
The link for the maps in Philly on the right over there in the sidebar-----> is no longer active. There are a bunch of other maps on that site to choose from but I don't see an easy way to print them. You won't really need them with the map above.
One more thing, we'll be headed down the Jail Trail to start, over the Glenshaw bridge and on down 837 to Mckeesport. A nice neutral start to the beginning of the trail.