For those that were curious. I don't actually have a rack with tubs this big. I built this for Joe with a design that I came up with for a smaller tub. However, I believe that a 1000 gram ball python could reside in that tub, however, it would need to be moved to a larger tub shortly. As far as the rails being on top of the shelf, the simplest explaination is that if you were to put them on the bottom of the shelf, you would have to make the unit wider, because the top of most tubs is wider than the bottom. If you look close at the picture, you can see that the tops of the tubs hit the side uprights. Hope that explains it a little better.