Bumping into someone in a shop, or a bus queue, or a pub, or at work and striking up a conversation, is a bit different from having someone plonk themselves down in the next train seat and starting to talk at you. I do find headphones very useful should the situation arise, but really it's a social no-no to start talking to someone who is pretty much trapped and may be too polite to tell you to **** off.
It's worth noting we don't know where the man and the woman mentioned in the original post were sat. It might have been the woman chose to sit next to the man because that seat was free, it might have been the man was the other side of the aisle or it might be he was sat opposite her at a table seat, we don't know.
Not sure why you consider a bus queue different to being on a train.