At this point, the Eagle has no dining car service between Dallas and San Antonio. I believe there is snack service in the lounge.
If the Sunset is on time you will have a 3 hour layover in San Antonio until the Eagle departs. However the Sunset has a habit of frequently arriving late into San Antonio, so if you end up arriving much after the Eagle's 8:00 departure time they will BUS you north to catch up with the Eagle, or perhaps all the way to FTW! Just be prepared for that possibility. In that event your SAS-FTW sleeper cost is refunded.
As previously stated, there is no dining car service between San Antonio and FTW/DAL, but the last time I rode they did provide a box breakfast and lunch for sleeper passengers - nothing to get excited about. Possible that might have changed to just comping you with something from the lounge car - I'm not sure. Also, the only comp drinks in the sleepers any more is bottled water, but there will be ice available if you want to pack your own. So be sure to enjoy the dining car while you can LAX - SAS!
Above all, keep a positive attitude, remain flexible, and enjoy your trip!
The eastbound Sunset was only two hours late into San Antonio this past time, and the switching was done during daylight hours (about 8:00 am), so the majority of passenegers were already awake. I've always made the connection to the Eagle, they've held the train as much as several hours waiting for the Sunset to arrive.
Our sleeping car attendant provided us with a muffin as well as coffee until we were coupled to the northbound Eagle and made our way to the lounge car, where we were also able to get whatever breakfast items we wanted. You sign a ticket just like in the dining car. Same thing for lunch... you can get any of the menu items available in the lounge car.
Also, we had coffee, bottled water, and juice available in our car throughout the trip... all day long, as well as newspapers. I think the only service item missing in the sleeping cars was the sodas, although I usually don't drink them, so I could have overlooked them.
If you are returning to LA on the Eagle, you will get a large cold boxed dinner after Fort Worth, since the dining car crew leaves the train at Fort Worth. I've always liked these dinners, they are quite good despite being cold.
The switching of cars from the Eagle to the westbound Sunset always takes place at night, and since I'm a light sleeper, it always wakes me up. The benefit of having your own sleeping compartment means you can make up for it by taking a quiet nap later that day.
Have a great trip!