I would always find people in aeroplanes less threatening than in trains.
Hadn't noticed that before but now you mention it, I think I have a weaker version of the same intuition.
I expect part of it's based on status of course, but part of it could be that it would be much harder for a mugger to escape on a plane. No crowd of people standing up to blend into, and no easy exits.
Also on some trains you have seats facing each other, so people get used to deliberately avoiding each others gaze (edit: I don't think I'm saying that quite right. They're looking away), which I think makes it feel both awkward and unsafe.
r/Fitness does a weekly "Moronic Monday", a judgment-free thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. I thought this seemed like a useful thing to have here - after all, the concepts discussed on LessWrong are probably at least a little harder to grasp than those of weightlifting. Plus, I have a few stupid questions of my own, so it doesn't seem unreasonable that other people might as well.