I'm from Boston. I saw spontaneous group conversations break out among strangers fairly often. Certainly, I would not think twice if it happened there. In eight years in Seattle, I have never seen it happen, not even once.
I once talked with someone from Seattle about our similar experiences riding the train in D.C. In mine, I was looking around too much for the young professional seated next to me. He removed an earbud to ask me if I had a problem, I said "no, just my first time riding the train here... So what do you do?" His response was scathing, "I mind my own damn business." She remembered being scowled at for offering another passenger a piece of gum (a girl with an iPod, earbuds, and newspaper).
Being from the midwest, I've taken the train in Chicago many times, without ever having an experience like I did in DC. I would say that commuters in Chicago tend to be comfortable exchanging glances with one another and making smalltalk, quite unlike DC.
DC public transportation does seem to have an ethic of space--the other week a guy moved when I sat down next to him--and reserve. That said, I haven't seen a lot scowling when one is intruded on. I will not myself initiate a conversation unless the person is next to or across from me and there is obvious reason to--reading matter or possible recognition.
The context people operate in around DC is that there are more unpleasant people that ride public transportation than you'd expect. I've seen my fair share around here in DC, and someone in my unit has plenty of stories to tell about late night shenanigans of passengers (he's a bus driver).
Good point; I was thinking of variance between countries, but countries aren't monolithic either. There might even be interesting variation city-to-city, or between urban/suburban/rural areas, or different socioeconomic areas.
I have the opposite second-hand report, though: a friend from California tried introducing himself to his new neighbors in Boston and basically got no response, as if introducing yourself to your neighbors was a weird thing to do. His general impression is that Bostonians tend to socialize, but grouped along very specific location/class lines, where you're a "foreigner" if you're from even as far as Somerville, let alone California.
I've lived in Seattle 7 years and have seen (and participated in) conversation with strangers on the bus. It's not impossible though it does seem rare here.
Coffee shops and dog parks on the other hand are prime stranger conversation spots. Oddly not bars though in my experience. I'll be moving to Cambridge in a few months so it's good to hear people there are more approachable.
I visited Seattle last year and went to several coffee shops. It seemed weird that nobody would look up if someone came in. Everyone seemed deeply absorbed in their book or smart phone or tablet.
I lived in the greater Seattle area for a dozen years or so, ending around 2003. Even early into that adventure, I remember hearing about how cold and unfriendly Seattle people were. I recall an article in a local rag, probably the Stranger, lamenting that people were lonely in Seattle because nobody would talk to strangers.
One small contrary datapoint: on my first visit to Seattle from Boston where I was living, I was walking on a downtown city street, uncrowded at the time, and an ordinary passerby made eye contact and said "hi". I almost stopped in my tracks, that interaction would have never happened in Boston at the time. There, it seemed that all strangers in public were to be perceived as threats (probably beggars) unless there was evidence to the contrary. This sort of (perceived) general friendliness was partially what motivated me to move there, that along with the mountains+water and being able to drive 20 minutes from downtown and being in the country. Yeah, Seattle folks, it really was like that on the highways once, not too terribly long ago.
Being from the midwest, I've taken the train in Chicago many times, without ever having an experience like I did in DC. I would say that commuters in Chicago tend to be comfortable exchanging glances with one another and making smalltalk, quite unlike DC.