They did it several times a day. This is a Williams trailer and the uprights for the rear upper deck are hinged at the bottom. When unlatched, they tilt out to about 30 deg. The decks above the big truck are raised and the truck is driven in. On earlier versions of these trailers, all the upper decks swung out. I dig one up and post it. Meanwhile, here's a sister truck with the cab and chassis units loaded on the tail.
GMC Cracker box; Insured #154 by
Dick Copello, on Flickr