Always Always Always get the first seating for lunch and dinner reservations. They will still have everything on the menu, and the dining car staff aren't grouchy and tired yet. Also, if you eat a large meal with the desserts they serve close to bed time, you won't be able to sleep. Trust me! Don't forget, all you're doing on a train is sitting and eating.
The food leaves a little to be desired. Back in 2006 congress demanded Amtrak lower all its food costs. This meant the elimination of real plates (so they could fire the dishwashers) and a switch from fresh cooked food to pre-packaged "microwave/convection meals". It's a damn shame.
However, Dinner is still pretty good. You can't go wrong with the chicken. Always ask for the steak well done, because it will come out somewhat rare anyways. If you ask for it rare, it will still be moving on your plate. I rarely hear complaints about the pasta dishes. The peanut butter chocolate torte for dessert is a killer. I've never seen anyone be able to finish it because it's so rich. I've taken bets with table mates that they wouldn't be able to finish it and I always win!
Breakfast is ok. They still serve grits!! The omelets are pre-packaged, but they're ok I guess. Scrambled eggs are powered egg mix. French toast is what it is.
Lunch is an ABSOLUTE DISASTER. The burgers are the same burgers they serve in the cafe car, they just put some Ruffles chips on your plate and a pickle. They'll usually offer a salad, and that's ok. There might be a warm sandwich offered, which will have processed cheese goo on it, stay away from that. But with lunch, you still get dessert.
And by the way, this crappy Simplified Dining Service (SDS) is in effect on all trains EXCEPT FOR THE EMPIRE BUILDER. The EMPIERE BUILDER still operates with REAL PLATES and CUPS, real waitresses/waiters, and REAL FOOD COOKED FRESH ON BOARD THE TRAIN TO ORDER. So, enjoy the food on the EB as much as you can.