Why would you initiate talk with me in the first place, when we're in a situation where I have not explicitly chosen contact with you? (say on a train)

Also reading something would be a clear signal (also to me) that a person doesn't want to get disturbed.

When I have to tell you that I don't want to talk, you have already disturbed me. So, taking the cues here clearly is on you, not on me, at least in my opinion.

Edit: To clarify a bit, I'm talking about places with involuntary social contact, like for example a train or a grocery store. I go on a train because I have to get somewhere, not because I want to interact with people. It would be a different scenario say in a bar.

If you are in a public space, I think it's totally fair game to initiate talk. It's also totally fair game to signal that you aren't interested.