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's less room for them to take a swing at me than on a train.
Someone who confronts me on a plane is less likely/able to follow me home, or to somewhere isolated, than someone who confronts me on a train.
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.