Park Junction is in the vicinity of 33rd Street and Girard Avenue in the Fairmount / Brewerytown section of the city, along the border of Fairmount Park east of the river. I believe you can see it from Pennsylvania Avenue and also where the tracks pass under Girard. It's not the best railfanning location from a safety standpoint, especially if you're going to be spending a significant amount of time there with expensive electronic and photographic equipment. I'm not sure what the best time of day is for traffic in the area but I would try to get in, get your shots and get out.
Glenn