Congrats RF on your new purchase. You are going to love your 120. I've had mine now for six months and totally love it. While it is simple to use, it will require some experimentation to "learn" how it cooks. Munchonu is correct, the rails just hang on the sides of the smoker from the top. As you use the smoker the rails will actually begin to "curve" inward, probably because of the heat over time, when grills are not in place.
Enjoy