zahavasdad,
I was once going to use one on a plane and the flight attendent stopped me from using it
A plane is a completely different animal from an Amtrak train. I’m surprised (and a little horrified) that your self-heating bag made it through the TSA security checkpoint with no problems. The trains on the other hand do not have a security checkpoint, and the range of activities permitted on a train is much larger than on a plane. I’d still ask the attendant in the train car first before using the self-heating MRE bag, but I’d be surprised based on my experience if the answer wasn’t “sure, go right ahead.”