Jerry, thanks for posting the info on the 6500 series flatcars.
Do you have any information on the 6600 series flatcars? Were they converted directly to ng flats from cut down stock and box cars? I have seen photos of standard gauge D&RGW flats that apear to have been converted the same way (photo was taken in Salida with ng high side gons loaded on them presumably for movement to Alamosa after the Monarch branch was SG'd). Or maybe these SG 'cutdowns' were the source of the 6600 flats?
Mark Valerius