You also have to consider medical issues. If the train were to suddenly come to a halt right when a guy took a bit of a hot dog, you can expect that bit to get stuck in his throat and begin choking. Unless a passenger next to him knows how to Heimlich maneuver, that person could easily die . . . a liability which the Metro can easily avoid by banning food aboard the trains.
I hope you forgot a smilie here...
No, that's how sorry of a state our justice system is in. One woman tripped over her own child, broke her leg, sued the store and won!