I will add onto Marks thread.
I was excited about these trailers, and wish Lionel would sell just the 20' 'pup' trailers as well, I don't need tractor cabs and that is the only was that it seems they are available.
Mark, I am going to go out on a limb and say that the box underside is a toolbox, since on the PFE/BAR the reefer units are shown at the front.
Lionel 40' trailer
Lionel 20' pup trailer - smooth side
Lionel 20' pup - ribbed
Lionel 20' and 40'
Lionel from fore to aft - Lionel 20', Lionel 40', Weaver 35', Atlas 45'
Pictured on a Lionel PS-4 50' flat
Lionel pup
Lionel 40'
Atlas O pines 45'