The “story” of the decline in passenger trains and development of railroad intermodal services (at least on the Southern Railway) is much more intertwined than I believe is typically understood. There is a considerable amount of information in the SRHA archives that details how the Southern wanted to get out of the passenger business but did not want to lose the revenues from the Post Office. The PO started the shift to regional distribution centers at the same time reduced passenger train schedules made using the railroads to move the mail less of an option.
Terminal mail handling facilities, and people, were a railroad expense passed along to the Post Office. The PO accepted the Southern’s offer to containerize mail “door to door” at PO facilities with drayage by the railroad. The process began with the Southern stopping passenger trains at its new intermodal terminals to pick up and drop off Southern owned containers on CTTX flat cars. As the Post Office developed its regional distribution system, passenger train schedules (not easily altered because of established departure and arrival times) became more of a problem for the mail, the PO asked if dedicated freight trains could be operated. (Bypassing passenger terminals with containerized mail was a serious revenue loss for the terminals.)
SR President D.W. Brosnan (known to hate passenger services and anything not seen as an efficient moneymaker), agreed even if it meant running very short trains. The railroad offered selected shippers the opportunity to ship trailers on those same trains and “intermodal” services in the Southeast began. (The first interline services (in the East) appear to have been between the Southern and the B&O, another user of CTTX flats. The PRR, via Pot Yard was less interested as they considered N-S traffic short-haul business.)
So…my question about Flexi-Vans. Were they promoted by the Post Office or did railroads simply see them as an effective way to carry containers? The Southern Flexi-Van flats were all sold (?) to the NYC and the Strick containers (not easily adapted for std PB) were welded to chassis to become trailers as the Southern expanded its “Rail-Highway” strategy