Re.P.P.S: I've found it possible, enjoyable, and even personally enriching to strike up conversations with random people on NYC subways. I'd imagine San Francisco would be similar. Here in Tampa Bay, however, striking up a conversation on public transportation is only interesting for a mental health professional practicing his diagnostic skills. A certain concentration of interesting people is required before one can have a chance at finding one who isn't busy updating his facebook status from his phone.