Riverviewer,
I'd expect that to be a very busy day for the Auto Train, therefore there will be plenty of car to unload. Assuming that the train arrives on time, you could be one of the lucky ones and watch your car roll off the train first around 9:50 is generally when the first cars start to roll off. But you could well be unlucky and be the last off, which could easily mean that the hour is approaching 11:00 AM before you're on I-95.
And again, all that assumes that you are on time and not either late or early.
Then I'd worry for you about catching traffic as you head north on I-95, especially as you get closer to Wilmington and the mass exodus starts for the holiday weekend.
Finally, the time that you arrive in Sanford will in no way influence when you car comes off the train in Lorton. It will influence what time you get dinner, as the middle time 7:00 PM usually fills up by 1:30 PM and often earlier. The 5PM is the next to sell out often by 2PM, leaving only the 9PM for those arriving later.
But how the car carriers are loaded at one end has no bearing on how they get unloaded at the other end. In fact, on my recent trip on the AT, I was driving one car and my brother-in-law was driving his family in another car. We both got to Sanford at the same time, checking in one right after the other. Yet in Lorton my brother-in-law's car rolled off the carriers about 20 minutes before my mother's car that I'd been driving.