I'm a pretend I'm asleep and already sprawled across both seats kind of guy.
There are certainly signs of social disengagement on trains and planes, but not to the extreme that Kim describes, and spontaneous camaraderie on these modes is much less rare. -I just wrote about my experience on Amtrak here. I think trains are the most social of all the travel options. Reason being, they have cars that are made just for such things. A smoking car, observation car, dining car, bar. When you are in these cars, it's perfectly acceptable for a stranger to speak to you. When you are in your seat, it's very much like the Greyhound rules.
I've not traveled by bus inter-city. After the account of the beheading, I'm not sure I want to. -Yikes.