[Bill Murray]

David:

I think you are on the right track in a roundabout way.
The photo I have is not interesting enough to post but it is of a trailing trailer so to speak which was used to transport amongst other things Higgins boats and various pieces of floating bridges.

My theory is that there are/were a number of consists that were either easy to bundle (telephone poles, concrete or steel pipes) or were items of somewhat large bulk such as assault boats, airplane fuselages, fire fighting lengths of pipe, and the like which were rather bulky but were capable of being transported without necessarily requiring a full trailer as described above. In other words, they provided their own strength to support the load.

Hope that helps a bit.
Bill