At the main-hall end of the tunnel, they have separate marked lines for Northbound Surfliner, Southbound Surfliner (but not separate Business Surfliner [grumble...cf. SAN]), LD Coach, and LD Sleeper. I haven't recently been there at CD boarding time, but I pass through every day at SWC late-ish boarding time. I think the theory is maybe that they board sleeper pax slightly earlier, but in practice people just seem to sit all over the place in the big banks of big chairs all around the tunnel throat, and then just kind of wander down when they hear the track assignment. IIRC LD trains operate from tracks 11 and 12. As others have suggested, you probably don't want to hang out on the platform itself egregiously early (not least because the track assignment may be different) (hypocritical of me, as I go and wait on the platform at 9 every day to wait for 785
), but a good compromise, IMHO, would be to go hang out in the tunnel itself (i.e. the one perpendicular to the tracks, not the ramps up to the platforms) somewhere near the area between the ramps for 9/10 and 11/12. Then listen carefully, and when they make the track announcement, which can be heard in this area (though sometimes indistinctly), wait a respectable minute or so,
then go up.
Anecdotally, I suppose they
may assign seats in the 'official' waiting area: there's a podium there that has a display of timetables on top, and underneath, at the back, a supply of blank seatchecks.