The only time I had a problem was when a couple of non-Japanese passengers figured they could remove my suitcase from the space behind the seats and put it in the area between compartments ...

Jib71: "What do you think you're doing with my suitcase?"
Passenger: [Plays dumb by responding in a language which I do not speak]
Mrs. Jib [Responds in their language - much to their surprise]
Passenger [In fluent English]: "Our bags contain valuables. We need to watch them"
Jib71: "My suitcase is valuable and if you touch it again, I will tell the conductor that you tried to steal it and we can discuss it with the police at the next stop"

... And then Mrs. Jib made friends with them and spent the next two hours chatting away. There are times when I wish she wasn't so good at getting along with people.