by C&O 15
If the Capitol schedule is anywhere close to reliable (Is it? I don't know) it should be doable. Arrival at Washington is scheduled for 12:24. So you ought to be able to switch auto racks and a few passenger cars off the tail end in time to leave Washington by, say, 1:30 or 2:00. That would put you in Lorton around 3:00. Add the Lorton passenger cars on the head end and the Lorton auto racks on the tail, and you're all set. It just doesn't give you a huge margin for error.