[...]And I think you'd have a hard time finding an airport caterer willing and able to prepare full service meals outside of major hubs. Restocking the Southwest Chief in Trinidad, Colorado? Good luck.[...]
I would think they would restock the Chief in Kansas City and Albuquerque, and the other routes at the appropriate major cities.
I also don't think the delays, unless they are extreme, are a major issue. If the restocking occurs at a staffed station (which is likely because the staffed stations are mostly in major cities that also have airline hubs and caterers), they could install a large refrigerator in the station, and if the train is late, the catering truck would just unload into the fridge, lock it up and leave. The on-board crew, perhaps with the assistance of the station staff, would load the train when it arrives. Walk-in refrigerators, such as those at any medium or large grocery store, could easily accommodate wheeled carts. The catering truck would just unload the entire carts and roll them into the fridge. When the train arrives, the carts could be rolled directly into the diner (if it has room), using a wheelchair lift if necessary, or they could be rolled up next to the train to transfer the trays of food. (Big trays about 3 feet square, holding many items, not individual meal trays.)
This is probably what they already do at the trains' originating station (LAX, CUS, NYP) or in the nearby yards where they prep the trains (Sunnyside, etc.) I think the goal would be to have fresher meals and not to have to store so many meals for up to 3 days on the train. I don't think the goal would be to offload the kitchen facilities so all the hot meals were prepared just-in-time at some midpoint in the journey. (If they want to do that, then just bring back the Harvey Houses, and stop the trains for 3 meals a day while everyone eats in a real restaurant.)
Many very high quality restaurants prepare much of their food in advance, refrigerating it or keeping it warm, and only cook the things that can't keep at the time the meal is ordered. All the breads, cakes and pies are baked early in the day, the vegetables for salads and sides are cleaned, sliced and refrigerated until needed, soups, sauces, etc. are cooked, refrigerated and warmed on demand, etc. All this would be done at the caterers, and delivered ready to be heated (if necessary) and served, and would rival the best restaurants if done competently and using high-quality ingredients. Only the main entrees would suffer by not being cooked from scratch in the diner, but they could and should still be of much higher quality than any TV dinner, certainly as good or better than any mid-level popular chain restaurant. If the meals were selected for being able to tolerate this treatment (e.g. no souffles or other fussy concoctions), they could be quite good. Also, there is nothing about this preparation method that forces the use of excessive salt or sugar in the recipes. And good ingredients are only a minor component of meal cost. Most of the cost is in the labor to prepare it, which is basically the same no matter what the recipe or quality.