Thanks Tink - I have read that your reservation can get bumped for military deployments (which I completely understand). Can you also recommend any other towns or areas where we might look? Thanks!
I think this situation could happen almost anywhere in this area since there are there are multiple military bases in the vicinity, and as many times as I've driven past that particular Comfort Inn I have never seen its parking lot full. I don't think a bump is likely.
But, since I am also a firm believer in contingency plans, here's an alternative or two:
First, I-95 runs right through Springfield, which you would reach about 5 miles before the Auto Train exit. There are several hotels in Springfield - Holiday Inn Express, Hilton, and Best Western to name three. But, Springfield can be very difficult to maneuver around since this is the area called The Mixing Bowl, with multiple flyover ramps and, if I remember right, 12 or 15 lanes of traffic. This is where the Beltway - I-495 - converges with I-95/I-395. I avoid this area, and I live here.
The second alternative is to pass the exit for the Auto Train and drive another 5-7 miles or so further south to the Potomac Mills area (Woodbridge) where there is a Holiday Inn Express.
http://www.hiexpress.com/h/d/ex/1/en/hotel/WDDVA/welcome. They have a small indoor pool, offer hot breakfast bar, and are right next door to Romano's Macaroni Grill (Italian restaurant chain), and On the Border (Mexican restaurant chain). Its a pretty easy off and on from I-95 at that location. From I-95, you would take Exit 158B. Turn left at the 3RD traffic light on to Telegraph Road. The hotel is on the right. After exiting I-95 and before turning onto Telegraph Road you will pass a Target on the right, if there is anything you need but forgot to pack. Actually, there are TONS of restaurants and shopping in that area, including a
Disney Store Outlet at Potomac Mills (a very nice, and huge, outlet mall), which is just down the road from the hotel. The next day, you would just double back the way you came to get back to the Auto Train.
I still think you would be fine with the Comfort Inn location, but I think this would be a good alternative. It just depends on whether you want to be closer to the Auto Train or have more amenities around you. Either choice is a good one.
We arrived in Lorton at around 1pm and just got quick lunch at Burger King. We checked in at the station at around 2pm and there is a little playground for the kids to play in and a little gift/snack shop as well. We walked around outside and watched cars being loaded and just generally took it all in. They begin boarding at around 3pm so the wait really wasn't that bad. Once boarded we dropped all of our stuff at our seats and headed for the club car for snacks. It wasn't so bad waiting to board but it seemed like an eternity to get our car upon arrival. I guess once we were off the train we just wanted to get going. 
In reality it was only about 30-45 minutes.
