I have a question. My name is Michael, and my son and I love trains. Problem is we live in bergen county new jersey, and while there is a slew of old train lines and old trains, there all on private property (owned by a company), and not so we can get a close up look. I'd absolutely love and so would my son (he's 3) to see what it's like inside the cab of an engine or a caboose, stuff like that. Any good train places around bergen county or nearby where we can enjoy our love of trains up close.
While not famialiar with New Jersey, I can recommend 2 places you can travel to, maybe make it a day outing or a weekend, depending on how far.
First is the B&O Museum in Baltimore, Md. http://www.borail.org/ I haven't been there in a long time but due to your location, it might be close enough for you to visit.
Second I have been to several times. Steamtown in Scranton Pa. http://www.nps.gov/stea/ Definately worth the trip.
I do recommend checking the schedule and plan around it so you know in advance if trains are running and what kind. Also if there is a big event going on and you would rather be there when it is not as busy, that helps too.
As far as active freight or passenger railroads, most do not allow riders anymore. They blame it on the insurance.
Mike