I take NJ Transit a lot. You're expected to just sit and be quiet. That's common courtesy on those trains.
Are you sure you're not in the silent car?
Even if you're not, usually it's people that think they are talking at a low volume, but are speaking louder than that. People are trying to sleep on those trains, people are trying to work, and even absent those two, listening to somebody else's conversation is annoying.
Even worse is people who talk on the phone. Talking to the person next to you at a super low volume on a non-silent car is probably excusable, but talking on the phone is 10x more annoying for some reason.