Peterbird,
I'd almost bet on at least some delays, so I would plan for that. That way if you do arrive on time, then you can be pleasently surprised.
Since you're bording in DC head for the Club Acela lounge when you arrive in the station. You'll find the lounge behind the Amtrak Information kiosk between gates D & E, ring the bell by the double wooden doors and just show your tickets to the attendant. This is a nice private waiting area, with refreshments, computers, TV's, and nice comfortable chairs. They'll also board you directly from the lounge, so that you won't have to stand in line at the gate.
Finally, you might wish to check out
these hints and tips from OTOL.
Have a great trip!