D. Scott Chatfield
That's an interesting document, but those cars weren't released to Trailer Train for use as intermodal flats. Those flats were Trailer Train classes F89CH and F89DH, and I don't recall any of those ever being used as pig flats.
In fact, at least two of those car numbers are specifically called out in the January 1980 ORER as wearing TTKX reporting marks, which was just a slightly different version of a hinged-deck tri-level rack. Since no new KTTX or TTKX racks were being built in 1978 that I'm aware of, I would have to conclude that the existing rack was just modified.
Most of the rest of the numbers are individually listed in January 1980 as ETTXs (fully enclosed "wide-body" tri-levels). This conversion did require the old rack to be cut off, and the carbody was converted to a "wide body bowl deck" and the classes changed to F89CHW and F89DHW. This was a major rebuild program as far as the flat car was concerned.