I have attched a copy from an Swanswood Garden Railways (SM32) site of the British design of WW1 WDLR wagons; these were 17 foot long and the bogies were common under all the WDLR (war dept light railway) wagons . There werer many variations, and all I think were 17 foot long.
As can be seen it is shorter than the photos in the first message on this topic; I cannot find any photos via google to match photo 1 but still believe this will be based (?) on a WW1 US Army design somehow, and is quite long, certainly by British narrow gauge standards (and most are MUCH shorter indeed!)
Neither can Google assist with reasonably priced books on the WW1 Light railways, it seems to be a very sparsley documented subject; not really surprising as the stock involved was meant to have a very short life!
The exception is the much illustrated Baldwin 4 6 0 tank, (by Accucraft) the full size 12" to 1 foot versions now being re-built, with one having been resurrected already, and the Welsh Highland have another! .
Flats were much used, as in narrow gauge rr's ,as they could & did take anything, and everything. The body could easily have been added later, if the basic flat was in reasonable order when bought.