One thing that hasn't been discussed much is that there is a limit on the amount of revenue that can be produced by food service cars on long distance trains. A Superliner train consist of 3 coaches and 2 sleepers has a maximum passenger capacity of about 300 people (72 per coach, 44 per sleeper). Of course, that's only if the train is full. This can be compared with the typical 8-car Northeast Regional, which has a maximum capacity of almost 500 passengers. (72 per coach in 6 coaches and 62 in one business class car,)
Thus, the most that a dining car could service during a meal period would be 300 meals. Of course that's only if the train is 100% full and everybody eats a meal in the dining car, so in reality, even if the food was excellent and service top-notch, a lot fewer than 300 meals are being served. There are 18 tables of four in a Superliner diner. Somebody with restaurant industry experience should be able to figure out how many customers a restaurant with 18 tables needs to serve to be profitable. The problem is whether there that number of customers riding the train at mealtimes day every day.
This is why I think the convenience store/fast food model wouldn't work. First, those types of eateries actually require fairly large staffing. I was in w Wawa today, and they had 5 people making food and 2 cashiers. This works for them because the place is on a busy highway with thousands of cars passing by, and hundreds or more stopping by to order food. I can't see how it would work for a train with a limited number of passengers, except it might be a good idea to have a good (if by necessity small) selection of grab and go items that would reduce the time demand on the cafe attendant, allowing for more sales per unit time. (Wawa has a large selection of grab and go stuff.)
It seems to be that the most cost effective solution would be airline style tray meals, pre-ordered and prepared off the train. These could be ordered from a contractor that is making similar meals for, say, airlines, and thus the marginal additional costs of preparing the relatively few additional meals for Amtrak passengers are relatively low. Of course, there would be problems from getting them from where they are made to the train, and string all the meals for a 3-day trip. Resupply during the trip might be possible, but Amtrak on-time performance can be so bad that the train might not make it to the resupply point in time for the meal.