I got to say, honestly, its just a greater respect for personal space and privacy.
Upon seeing someone on a train, I don't assume they have nothing to do. They could be thinking about a big presentation they have to give at work, or their partner who's been cheating on them, or this great date they're going to have later in the day. They might be an introvert, and uncomfortable with other people. They might be figuring out when they can book in their next Doctor's appointment.
Why would I try to interrupt that because I'm bored? In an office building at work, when you see a co-worker working hard on a problem, do you saddle over and try talking to them because you're bored, then complain they're rude when they say they're busy? No? Same here. You don't know what someone else is doing. If they want to talk, they'll lock eyes, flash a smile, and approach you. Otherwise its assumed everyone has their own business to take care of.
Its plain respect for each other; let those who don't want to be bothered because they're uncomfortable with other people, busy thinking about something, or are otherwise engaged do what they need to do and leave them to their own devices. If you want to talk, find someone else willing to and talk to them. No one owes you a conversation because you are bored. That's just weird, the thought that I owe you, someone I've never met before and will never see again, something while you owe me nothing, just because you're bored and want to talk [And no, talking back to me isn't what you owe me - I don't want that. If you owed me anything, it'd be my own privacy]. If you want to talk to someone, pull up Facebook on your phone, go to a pub or cafe, or otherwise find someone else who wishes to converse. Don't try to force everyone around you to be your personal courtiers, hanging on your every word because you are oh so mighty and self important.
See, its seen as being more self important and arrogant here if you strike up a conversation with someone without any sort of permission. You think you're better than them, and that they owe you their time they might need for something else. They're not entitled to some down time, some peace and quiet. They must talk to you because you are bored, and you are all that matters.
Apparently things are seen different where you're from.
In general, the best path of action is to understand the local culture, and try to fit in. Respect different people, and their different approaches to life, and when in Rome, do as the Romans do.