I received these cars yesterday and I am somewhat puzzled. One car has the pedestals for 20' and 40' containers so no problem there. However the car labeled Mark V has no pedestals. It has a deck over the trucks and two small decks in the middle. Looking online shows me pictures of two 40' containers on the car with the center platform lined up with where a truck would be if the container were on a trailer. This seems unsafe to me. Would the container be stable this way? I have been led to believe that you had to support containers at the corners, nowhere else was strong enough. All my trains are packed away now so I do not have any containers to test with, but will that platform in the middle fit into the notch in the bottom of the container model? If not, how are they mounted.
The later modifications included standard container pedestals. The Flexi-Vans had two mounting points, one at the rear of the trailer body on the pivoting turntable, and one at the king pin.
These cars offered by Trainworx have end turntables. They rotated 90 degrees and a trailer was backed into position to slide onto the turntable. Part of the procedure was to remove pins holding the highway wheel bogie to the van body, and allow the box to slide off the wheels and onto the turntable.
The van body was then rotated on the turntable, with the king pin locating into the smaller center brackets on the spine of the flatcar.
If you'd really like to dive in deep, I've uploaded a copy of the NYC Flexivan Loading Manual here:
http://cgwrr.com/FlexiVans/NYC/flexbook_handbook.pdf