I think the area around the zoo and from there south towards 30th street station would be your best bet. One word of caution though. Try to stay on the east side of the river. The neighborhoods on the west side south of the zoo are not the best (Powelton Village and Mantua, I’m sure you’ve heard those names mentioned on the news). I know CSX runs through that area, but I’m not sure of NS, but the tracks are on both sides of the river so it should. There’s even a tunnel that goes under the Art Museum.
http://www.panoramio.com/photo/28951933?source=wapi&referrer=kh.google.com
http://www.panoramio.com/photo/54440465?source=wapi&referrer=kh.google.com
If you have Google Earth, you can get a pretty good map of the railroads in the area. I typed in 30th Street Station, Phila. Pa. and got a pretty clear view of where everything runs. My GPS even identifies the RR that owns the tracks.
Further south and east of where the stadiums are, there’s an intermodel yard where they off load the containers from the ships to the trains Next to the old Phila Navy Yard. There’s any number of side streets off Pattison Ave. to access that.
http://www.panoramio.com/photo/64666141?source=wapi&referrer=kh.google.com
Further south below Chester, PA, CSX has an auto train off loading site. It can be seen and accessed from Rt 322. I’ve often seen the auto carriers sitting in the hole along I-95 there waiting to get into it. I’ve seen the Tropicana orange juice train run through there, but that’s a hit or miss situation.
Most of these sites can be seen from the Schuykill Ex, or I-95 as you had towards the Airport. Those spots are the ones I’m most familiar with since I grew up and worked in that area.
The areas mentioned by Dave are also very good spots, but quite the drive from Ft Washington area.
Almost forgot. Here’s a Google Earth shot of the 30th Street station area. On GE you can zoom in on it and follow the tracks.
(http://i253.photobucket.com/albums/hh58/rgseng/Philly01a.jpg)
