I boarded the Coast Starlight in SJ last week, and as soon as we got going the dining car called for anyone that had just boarded and were in sleepers to come down for dinner. The train was on time that day, so we boarded right at 8:23.

I also asked what the policy was for how late they will serve dinner and was told "as long as you are on the train before 9:00 PM you will be able to eat in the dining car" by a dining car attendant on the California Zephyr. He also advised that if it was close (say, after 8:30 or so) to alert your sleeping car attendant immediately upon boarding so they can make the arrangements for you for a meal before the kitchen shuts down.