I installed the Thule Yepp Maxi child seat and the caboose on the RadWagon 4. I installed both the inner and outer rails of the caboose.
The Thule Yepp Maxi child seat will not fit between the inner rails. So, I had to take out the inner rails. The rails can be a bear to install and remove. On mine, the screws didn't want to go in and then did not want to come out. I believe it is because of the Loctite on the threads.
So, just a hint. Don't install the inner rails if you plan to install the child seats. Check the width first.(//)