When I was in Japan, I took the trains like crazy. Because I didn't speak much Japanese and didn't know anyone there, I was pretty desperate to talk to people. Everyday I would sit on the train and wonder why no one was talking to each other even though they could. It amazes me that strangers who share a language are able to talk about even the most complicated topics but generally don't. I felt that those people were really missing a great opportunity because they took there ability to express themselves and understand other for granted.
Now that I'm back in the old USA, I never talk to people on the bus and am back to not wanting to as well. Public transportation is a kind of public gathering by accident. While not intended to by a meeting place for people it could very well be.
In olden times, there was this thing called the commons. The commons were plots of land governed by some really old unwritten rules and are interesting to people today because the land had a special place in a legal system based on property rights. It was thought that the community (or sometimes God) owned this land and people in the community all had the right fish, gather wood, graze cattle and such on the land. I think that in modern times those types of common spaces which were originally utilitarian became a gathering space like the town square or open amphitheatres and such.
Despite not having much of a historical background, I feel like public transportation is today's commons in a cultural sense not a legal sense.