Here's my experience with (attempted) theft on a train:

I once was on a MARC train at DC Union Station. Some train cars have electrical sockets, so I plugged in a bike light I had since I'd be taking a bike for the last part of the trip. The train hadn't left the station yet. I was standing near the seat with the socket. Some unassuming looking guy was walking through the train car, like probably 100 did before him, when he grabbed the light, unplugged it, and kept walking. I immediately confronted him (I was in his path) saying something like "What are you doing?" Without a word, he handed me the light and walked off the train. I found a conductor like 15 seconds later and they called security, who apparently detained the guy.

This guy was way more brazen about stealing something of little value than I had expected. I was standing near the seat and watching it! I guess he didn't expect me to be the owner.