On a train you can generally move away from someone - much more difficult on a plane. While I didn't feel unsafe I have pretended to be asleep on a 3hr flight to avoid conversing with a group of drunken individuals sitting around me.
Indeed, I moved away from a group of people on a train (drinking at 09:30) who were using colourful language as I had small children with me.
That wouldn't always be so easy on an aircraft.
As to people stealing things, it's not overly likely in either setting and chances are good struggle to find out who it was on a plane with 200 people on, given that there's got to be evidence. Do they have CCTV? Even if they do, chances are any police case would involve trying to contact people spread around the world.
As such I'm not convinced that there's much difference and the risk is fairly low anyway.