Never done the Day with Thomas but I just wanted to mention, as others did, to be warned about the traffic. We live near and I used to work in Strasburg, really neat place but it doesn't handle traffic well. Just be prepared to deal with lots of people and kids. It's also right in Amish country too, nice treat for those who aren't used to seeing them all the time. I would agree with the idea of bringing a picnic lunch or at least snacks.
Have fun!