Here's what NSK has to say about these -
As trailer-on-flat-car (TOFC) service evolved in the late 1950's, many roads began trying to cut costs, opening the door to new ideas. Among these was the Flexi-Van system, first tested by the New York Central in 1957. Designed to speed loading and unloading, the Flexi-Van design used a special turntable and a trailer (36' or 40') with a removable wheel assembly called a "bogie". In operation, the trailer on its bogie was first aligned with the turntable and backed into place. The bogie was then unlocked and the trailer placed aboard the Flexi-Van car. Once in position, a pin locked the trailer to the turntable, which was then turned to the loaded position using the on-board hydraulics.
The NYC introduced its Flexi-Van cars and trailers in 1958. These were low profile skeleton cars, designed to meet clearance restrictions on the NYC while carrying two trailer units. Early cars handled only 36' units, but as 40' was quickly becoming the standard length for highway trailers, the later Mark II models carried a 36' and a 40' unit. Some Mark III and Mark IV cars built from 1961 to 1968 carried two 40' units. It is the Mark II car that N Scale Kits has produced.
Lighter and lower than standard TOFC cars, the unique Flexi-Van design proved well suited for high-speed operation and many cars were rebuilt such that they could be moved in both freight and passenger service. The system eventually proved popular for handling mail. A number of roads used the Flexi-Van cars including the CB&Q, IC, MILW, and WP (et al). The rapid rise of containers, the acceptance of industry-wide methods for moving trailers on flat cars and the unusual design of the Flexi-Van system ultimately brought about its demise in the early 1970's.
The N Scale Kit car is cast in lead-free pewter, and therefore has a very low centre of gravity and sufficient weight to track very well even when not loaded. The grabs and steps are in etched brass. The kit comprises a single Flexi-Van spine to which all the fittings, including the container turntables are glued/placed. CA is recommended. Simple hand tools are required for assembly. Decals are available for NYC and Milwaukee Road, the main owners of the these cars. Price $23.99.