Yeah, in LA they only preboard sleeper pax on long-distance trains (plus special assistance pax), not Business Class on Surfliners.

But I can't recall ever having stood in line to board a Surfliner in LA. No one needs to see a conductor in advance to get a seat assignment. There's no controlled access to particular platforms; they're always open to anyone who wants to access them.

Barring trackwork or extraordinary circumstances, Surfliners always board from the three platforms of tracks 7-8 / 9-10 / 11-12. I usually go straight to the tracks 9-10 platform to determine where the train will be if it's not already in the station (this is usually possible without having to ask anyone if you know what to look for, but going up too early may raise hackles with security guards or Amtrak staff).

More simply, next time you could sit comfortably in the station and wait for the boarding announcement. Then simply follow, or walk ahead of, the scrum from the boarding line down the tunnel to the appropriate platform.

Either way, given the setup in LA and the lack of in-station ticket collection for Surfliners, there's really no reason to line up for Surfliners there.