LM, I dont think it is retractable. I think it has been added to the 3D assuming how it looked. I have seen pictures of it on wheels and on skis, but the tail wheel not visible.
It has two sets of wings.
And sorry, I missed one of your questions in the beginning: No periscope.
And skytrain sigle seat, behind the wing and nothing interchangeable (as far as I understand).