I am a regular traveler via rail. I use local rail AND national rail. Most everyone I have met have been very pleasant, welcoming, and quite willing to have conversations. I have had conversations about almost everything you can imagine, except things that were obviously topics that two new people never talk about.
I have met some nasty people with negative attitudes, and I have to say that the worst of them have been from far northern California.
When it comes to being social on a train, I stick to some very common sense rules that go like this: stay away from any "quiet cars" or other established quiet zones. I let people get settled first, read their body language. I make eye contact and SMILE...if a smile is returned, a second look is given, or other interest shown...I speak up and say something pleasantly engaging. A light sense of humor that comes from a commonly shared human perspective will almost always work, and is something you can always build a wonderful conversation on.
On trips across the nation, I stay away from engaging people late at night unless it is in the view liner, and even then I am careful about that. I have seen people meet up and in a matter of 30 minutes or so, end up engaging in adult activities on impulse. You'll be surprised what you'll witness on long-distance trains!
I have so much more to say on the topic from personal experience.