Hi ValC. Welcome to the forums. I hope you are still looking around and find this site useful.
You will be boarding Train 4, the Southwest Chief, EASTBOUND from San Bernardino on your departure date, scheduled for 7:59 PM. There is pretty good likelyhood that this train will depart on time as it is only the third stop on its 2,265 mile journey from Los Angeles to Chicago.
I would take an opportunity, if it is convenient, to go out to the station around the scheduled departure time before your departure date to get a feel for the station, the trains and the movement of people. The big beautiful station is a museum now and only a small section of it is a waiting room. If you get there early enough, you may have time to enjoy the railroad museum. You will notice that to the left of the station are several platforms and tracks that stop at the sidewalk. These are called stubs and they are the end of the line for many Metrolink commuter trains. You need not worry about these trains. They are a mix (right now) of siver and white train cars. They will have METROLINK clearly marked on the front and side. Go past these stub tracks and come to the track side of the grand station. This is where you will likely wait for your train. The Southwest Chief will arrive from the left. Look for the ALL silver train with lomotives that clearly show the Amtrak logo on the front and sides and the coaches should all say Amtrak on the side with red, white and blue stripes along the side. The platform is short, so they may make a double spot rather than use the Metrolink platforms. I've gone through there 4 times, but I honestly don't remember what happened.
At any rate, when Amtrak arrives (remember, all silver train with red white and blue stripes on the side of the double decker coaches that have Amtrak written on them arriving from the left) you'll see a couple of official looking people hanging out of the doorway. They may ask you "Coach or Sleeper" so be ready to answer. They'll either tell you where to head or ask you to wait. Then, they'll ask you where you're going. Tell them your destination station and they'll direct you to a seat.
Again, I think it may help if you go out there a couple days before hand and just watch and see what they do. Arrive around 7:45 and just enjoy watching the activity. The more info you have, the better armed you'll be. I hope you enjoy your experience. This is probably the 3rd or 4th of the greatest train rides in North America, and I hope the crew is top notch for you.