As mentioned earlier, the second side gets screws. I drilled two holes and attached the second side with screws.
Once I got the cap moulding on, it was now time to glue on the top, bottom, and the sides. Actually, only 1 side would get glued on. The second side gets screws so you can put it on and take it off to get the cars in. Using some wood glue and clamps I glued the top, bottom, and side on.