You say you are traveling seven hours, so I suppose you are going to Birmingham? In which case lunch would also be served.
Many people, not just railfans, find eating on the train to be kind of an enjoyable experience. Real food, real tables, watching the scenery go by. Unless you are in a party of four you will probably be seated with strangers. And that can be some of the most fun of a train, the interesting people you sometimes meet that way--not always of course.
So consider getting at least one meal on the train, not just a water and an apple. And of the two meals I would recommend breakfast, the luncheon menu is not so special . Breakfast and dinner are the "biggies" in train meals and most likely to be enjoyable.
You will have to wait to go to the diner when they announce it as being open. It will not be open the minute you leave New Orleans. They have to collect tickets first.
But if you are absolutely sure none of this is for you, then yes, you can buy snacks in the lounge car, next door to the diner. But be flexible!
As I say, many people really enjoy the novelty of a meal in the dining car.