This is a very interesting thread, great exchange of information. I would love to see someone come out with a book or really extensive article on Central's development of Flexi-Van instead of yet another book about the Twentieth Century Limited
or Grand Central.
The little I can add is this- I recently did some research into the brief William White era on NY Central, roughly 1953 through mid-1954. White was planning to start a piggyback service on Central and even ordered some piggyback flats which were delivered, iirc, in late summer 1954. The service was to be provided on certain lines with the New York City terminal on the old West Shore at North Bergen.
By the time the flats arrived the Young-Perlman team had taken over, in July 1954, and the piggyback service was one of the first things they reviewed. During a press tour at Weehawken around September 1954 Perlman told the media the piggyback plan was under review and would not start as scheduled. When asked why, Perlman said they wanted to "make sure we offer the best service we can," or words to that effect.
In discussing this with a longtime industry person, from the marketing side, he told me he thought Perlman didn't like piggyback because Central had too many low-clearance routes where TOFC would not be able to operate. One important route with a major clearance problem was the Hudson Division into New York City. Thus they came up with Flexi-Van which could be offered to all of Central's shippers, unlike TOFC which would have been available on only certain routes.
In this person's opinion, what was sold to the public and the industry as a great innovation, a better way of hauling trailers, was really borne of necessity. That Flexi-Van solved Central's clearance problems and the road put the best possible face on it.
FV did reduce weight and wind resistance, and if you go back and look at Perlman's public statements he always emphasized that. But true enough, at least in public, Central never mentioned TOFC wouldn't fit on some of their routes. I guess they were too smart to do that. So there was a lot of salesmanship involved too.