If the train starts in Richmond it's likely to be on time. If it's coming from further south, there's a greater chance that it's late. I've found Amtrak to be reasonably reliable between RVA and DC, but it definitely isn't as good as it is north of DC.

If you want to do the train, it would be better to take it to Alexandria instead of Woodbridge. Not all trains stop at Woodbridge, but they all stop at Alexandria. Also, the Alexandria station is right next to a Metro station, so if the slugging option doesn't work out you do have the Metro as a backup. I would imagine public transit from Woodbridge is quite a bit worse.