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.
For comparison, here's what I come up with when I introspect about my intuition:
The planes I'm on usually have higher people density than the trains I ride.
People seem more likely to step in if a fight breaks out on a plane than on a train. (Although I wonder why I believe that, since I've never witnessed a fight on a plane. Maybe I'm influenced by point 1. I guess fliers are also quite proactive nowadays about piling on people who get violent on planes.)
Passengers on planes are screened for weapons before they board, and when they're on-board there'
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.