Republic at Farmingdale NY (KFRG) is closer NYC to than PHL, and Sheltair will drive you to the LIRR station. That said, the FBO's at TTN will probably do the same for Amtrak. And PHL is south of Princeton, not on the NYC side, so if you're going to do that, you might as well land right at Princeton (39N) and let your friends drop you at the RR station. As for Linden, between the airspace, NY TRACON, and the traffic, you'd best be on your "A" game, but it's not anything a competent Private Pilot shouldn't be able to handle. Just study and use the NY Terminal Area Chart, read the A/FD, mind your P's and Q's, and stay alert.