For comparison, here's what I come up with when I introspect about my intuition:

1. The planes I'm on usually have higher people density than the trains I ride.

2. 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.)

3. 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.

4. 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.