The Autotrain will probably never go north of WAS for other reasons than those already stated in this thread.
Height: Although I could not find the specs on the auto racks, they are about 12-18" taller than the Superliners (16'2") Since Amtrak limits heights for patron's cars for tri-rack is 56" this probably about right. So no Amtrak NEC raceway (15'6") for them. It would be all CSX all the way. They would have to avoid direct route through Baltimore as the tunnel CSX uses there can't even handle their own double-stack freight.
Turn-around: Current turn-around is 7 hours to unload, clean the train, restock, inspect equipment, fuel, load, assemble and test. If you run it 4 or 5 hours further north, there would not be enough time without adding 2 more trainsets. (unlikely) Beside that there is no room at Lorton for another train. As it is there is barely room for one.
The Potomac River: CSX owns the only rail bridge across the river east of Harpers Ferry,WV. They allocate a certain number of time slots to Amtrak each day. Between all the Amtrak services that run south of WAS, there are no more slots available.
Crew: The Autotrain uses 2 crews each way. If you lengthen the run from 17 to 21/22 hours, a second crew change would be needed if they run late. Also if they decide to use Viewliner equipment to access the NEC they will need crews qualified for it, more cost.
Food Service: The added time would probably require an additional lunch. Amtrak says they lose money on food service (I don't believe it) and every meal on Auto train is included.
Speed: The only way to get Autotrain north of WAS is for Amtrak/CSX to build a new 2-track Potomac bridge (separate freight and passenger) so they can run up the NEC using new shorter height auto racks and Viewliner equipment (with engine change at WAS). They would not benefit much from the higher speed track on the NEC as the auto racks are only designed for 79 mph. (CSX limits racks to 60 mph system-wide) The NEC is PACKED with trains in the late afternoon, so a 4 pm departure #53 from the New York area would be suspect. (Acela folks won't like getting their speed cut in half by a 70 mph Autotrain blocking the route)
Length: Since Viewliners do not handle as many patrons as Superliners, they would have to make the train longer to serve the same # of patrons. CSX limits the train to it's current length, so that's no go.
Have a nice day.
Harry