I don't think running connecting buses will be very successful. Too much hassles possible.LA Union Station would provide direct connections to the LA subways and light rail transit system. I would think X-Train would much rather operate from LA Union Station, but either it costs too much or Metrolink won't let them because of capacity constraints at the station and the tracks to LAX.Actually, Fullerton also connects with the Surfliners, which I presume would be (by a wide margin) the main connection concern. Particularly if DX/XW has a stop in Palmdale, there's not a chance they'll get traffic traveling from further north.Another reason for this train to run into LAX would be connections to Amtrak. Might not be much but could be something.
Fullerton is part of the Metrolink commuter rail system, but a quick check of the schedules show few Metrolink trains on Sunday and none after either of proposed Sunday trains arrive at Fullerton. The second train on Sunday is nominally set to arrive at Fullerton at around 10 PM, which is before the last north and southbound Surfliners go through Fullerton, but there would be little margin for the X-Train to run late. The X-Train business plan is probably figuring on most people driving to or parking at Fullerton. On the other hand, they may be planing to run several charter buses to Fullerton from LAX and several other locations in the LA area as feeder connectors to the X-train.