We took Auto Train from Lorton VA this month and saw two automobile carrier trucks leaving the terminal loaded with automobiles. Does Amtrak haul automobiles without passengers when they have space?
Only thing I can figiure out is that some auto leasing company or auto dealer had one employee ride the train for each car handled, so there was no violation of Amtrak's deal with the frieght railroads. They probably needed the cars moved in a big hurry and decided to spend the money.
Or---It was a CSX freight move and CSX asked Amtrak to handle it as a special case.
This is not a new policy. Automobiles have been handled in this way for a few years now. Not on every trip, but it does happen. I don't know how it is arranged, since I don't have anything to do with ticketing, auto loading, reservations, or any other department that would be involved. Your best chance for a good answer would be to contact someone in station services/ticketing at the Lorton or Sanford station. Going outside the Auto Train "family" is probably a good way to get unreliable, inaccurate info. That's a good rule of thumb for just about any questions regarding Auto Train.