I've had no problems getting on bustitutions with an unreserved thru-ticket stub. In fact, since you have to call to get AGR tickets that include a bus portion, I just order tickets for another day before so I can pick them up. You shouldn't have a problem.
The Del Mar trains they run (opening of the Del Mar races, etc) often are double consists of Pacific Surfliners; literally two trains stuck together, putting a cafe car on each end.
I think they've turned off the Wifi. I overheard a conductor say that it had something to do with interference going through camp Pendleton or something.
They're also long enough for a double-length Surfliner (two consists stuck together, an engine at either end). Those trains are the best, because they have two cafe cars.
Google Maps shows how long Anaheim station is (they even have a train!). If you were at the "wrong" end and reading a book it could be that you didn't notice the train until after it had basically finished debarking people.