This link should help you out, at least with frequencies:
http://www.parailfan.com/csx_trenton_sub.html
Also at Woodbourne, the NS Morrisville lines runs overhead as well, so you may as well use this page as well:
http://www.parailfan.com/ns_morrisville_line.html
As per traffic, it's an active SEPTA line, so you will see plenty of SEPTA trains - it's on the CSX Trenton Subdivision, and depending on what time you go, you'll see at least one CSX train if you hang out for a few hours. If you go in the afternoon, mid-late afternoon, and stay for two or so hours, you should see at least one CSX train, and if you're lucky, a NS train overhead as well. Search on Youtube, you'll see videos from some people with the types of activity you could see there (plus my newest Woodbourne video may not be too far behind). Good luck!!