The last time someone I didn't know tried to talk to me on public transport I moved to another seat.
Hah! As someone who used public transport for most of my life, I've suffered through more than my fair share of narcississtic self-obsessed extroverts who don't get the hint.
And some people's tolerance for tedious conversation can be higher or lower. Mine's pretty low, so I know where the P is coming from.
What's really rude is to waste people's valuable time and peace of mind.
No I didn't ask people to stop talking to me. The point I'm making is that I often run into people completely impermeable to social-cues, body-language and other hints.