Are you doing a one-night trip or long day trip and starting and ending in a city with lots of food choices? (Like Philly to Chicago, for example?)
If so, do you already have some favorite allergy-free menu items from places there that list their ingredients and could you pick up something to take on the train and then get something from a similar place when you arrive at your destination?
I don’t have any food allergies and have pretty much a cast-iron stomach, but after a day of train food, even I feel slightly uncomfortable after all that salt and sugar and fat.