It's not that the trains are so full of drunk homeless people, but that trains are affordable and also conductors aren't going to physically throw out a person that could get violent. Planes you don't really get on without a ticket. An entire class of people are filtered off the plane because of that.
And anyway, said group are a minority, but they're the only ones who randomly talk to strangers most of the time. Everyone else minds their own business in my experience.