Passengers seated on BART generally don't talk to each other, and people prefer it that way. I know I do. On occasions, when BART is really uncomfortably crowded, we make one-sentence comments to each other, like, "it's going to be a long ride", or "don't think it could get worse than this." And even more seldomly, people would ask me, "is that a good book? I've been thinking of getting it." I would then say, yes or no, followed by a one-sentence reasoning. I don't mind yes/no questions so much, as long as they don't have a follow-up question.
I generally don't want people to talk to me. I've been reading a travel guide on Eastern Europe (book is called Travel Guide to Vienna, Prague and Budapest), and for some reason, it has drawn a lot of unwanted conversations. People would ask me, are you going to Prague? I would bluntly answer, YES, and think to myself, "duh, it says that on the book" and wish that they won't talk to me anymore. Then, they would volunteer comments like, "oh it's beautiful there, you will love it!" or "Oh, I always wanted to go there!" or "make sure you see X or X". vSome people would even go a step further and ask me, "when are you going?" or "where are you staying?"
Don't they get it? I don't want to talk to them! I don't want their opinions and I don't wish to share my travel plans and details with them!! The worst part is, people who try to start conversations are usually the less normal (don't want to say abnormal) passengers.
I see people with Spain or France travel guides and I don't ever bother them and volunteer my personal travel experience! BART is just not the place to have conversations. If they need travel tips, they can go to Tripadvisor, Frommers or whatever! They don't need it from a complete stranger.