At lunch today we were discussing Metro conversation, and what constitutes acceptable chatter.
Ask about something the person is wearing (hopefully in flattery), asking for directions, comments about how someone just made it onto a train, or about faults in service (slow train, hot train, crowded train, etc.). If these conversations turn into greater ones, that's fine.
Just about anything else, especially comments about religion, work, kids, etc. that seemingly come out of nowhere. It's not cool to involve people in your personal life out of the blue.
Now, these rules are general. Me, I like talking with people and learning about what makes them tick. As long as you know when to shut up (like after 18 minutes of yammering) or when I'm done talking to you because I'm tired / in a bad mood, we're fine. Let's chat about your tattoo, or your child's addiction to video games, or the book you're reading. You have my full attention for the next two stops, because that's when I get off.