Depending on the crew, they may only give you one seating time for meals in the dining section of the Diner/Lounge car. I took the Cardinal eastbound from Chicago to Baltimore this past Thursday/Friday and that's how they handled it. No reservations--just a "come and get it" call.
I wish it could have been a little more flexible, but in talking to the staff, I understood the dilemma they were in as the LSA was handling the Cafe side of things and the dining steward was handling both cooking and table service duties.
That said, they were able to accommodate sleeping passengers who wanted to eat in their rooms at different times since the sleeping car attendant handled the food delivery for that arrangement.
Rafi