Daniel Mitchell wrote:

Just FYI. The old ROCO 6-axle flats are BOTH 1960’s vintage U.S. Army prototypes. The regular flatcar was for general use. The depressed-center flat is the transport carrier for the M-103 Heavy Tank (though it may well have been used for other things too).

For more about that Roco flat car and its actual origins, you might wish to read my blog post at:

https://modelingthesp.blogspot.com/2018/12/upgrading-roco-flat-car-part-3.html

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