If the train is on time into San Jose, then you can eat in the dining car after boarding. If you have a good car attendant, he or she will have made a reservation for you. You can check historical performance at ASMAD. Unfortunately, there is a hole in the recent data due to unscheduled maintenance (a bridge replacement in northern California) followed by scheduled maintenance (track work south of SJC) both causing suspensions of service.

If the northbound Coast Starlight is like other long distance pre-COVID trains, and that's two big IFs, then the crew will probably have an abbreviated supper service ending about two hours before arrival into Seattle. If the train is on time, of course, that means an early meal.

I would suggest you call Amtrak, but I have little confidence that the phone agent would know what should happen and I have much confidence that the crew would feel free to deviate from what should happen. I say that without snark but with sadness.