It used to be you checked in at the info desk and they assigned you a dinner time. But, someone said they aren't doing that any longer. Don't know why. It worked very well for us a couple of years ago.
They may have dropped the checkin, but dinner is still served to those boarding out of LA and I think Fullerton. Board any further down the line and you miss out on dinner.
I was on the SWC out of LAX a few weeks ago. Yes, dinner is served to those boarding at LAX and I think Fullerton. Soon after leaving, the dining car attendant comes through the train -- sleepers first, then coach -- taking dinner reservations.