Hubby and I are (he is...I was) both tour guides for this tour. It starts at 7:30 (they will ask you to be there by 7;15) and it ends between 10 and 11 depending on the size of the tour (max is 20 people) and any delays that may occur with the trains themselves out at the roundhouse.
Children have to be at least 10 to take the tour.
As for transportation. It all depends on where you are staying. Magic Kingdom buses start running from the resorts to MK around 6:45am (all are on line by 7am). If you get out to the bus stop early enough to catch that first bus you will be fine. You could also get a taxi, if you don't mind paying the fair, just make sure that you tell the driver you are going to the Contemporary. If you say Magic Kingdom they will drop you off at the ticket and transportation center, if you say Contemporary, you can walk across to the MK entrance.
Some things to remember when you book though. The speil that the CM will give you is the generic tour speil. They will tell you buses do not run this early so you will need your own transportation....they will also tell you that cameras are not allowed on the tour. As mentioned the train tour allows cameras, there are even special locations set up just for pictures, so make sure you bring the camerea.
The tour is a really good one. There is a great mix on the tour. There are train buff that know more than the tour guide

, train fans that want to know more and those that are along for the ride...honestly it doesn't matter what category you fall into, I have never had anyone on my tour that did not like it. Usually even those that were just along for the ride leave with a big smile and make comments about how much fun they had.