Hi Martyn,
The display in Chama of the pipe gons shows why they were converted, i.e. to carry pipe that was longer than 30 feet. So with that in mind, here is a little history behind the 30 foot pipe gons and idler flats.
The 30 foot gons were originally high side gons that had the ends removed and the brake staffs moved to the corner. This modification took place in the early fifties at the begining of the Farmington oil/gas boom. A fair number of high side gons (100+) were converted this way. There is a story behind the conversions that is too lengthy for posting here, but that info can be found in Dr. Robert Sloans excellent book
A century+ten of D&RGW Narrow Gauge equipment.
The idler flats were "kitbashed" 12"=1 foot style by the Grande out of stock cars, box cars and high side gons. These idlers were made by simply sawing off everything above the floor and shortening the brake staffs. The ex-high side gons had short lengths of used rail clamped to the deck to reinforce the frame, even so many of the ex-high side gons idler flat cars ended up broken in two from slack action and the fairly long trains (70+ cars with helpers).
The ex-stock and box car idlers were a little more robust and had old rail clamped for the entire length of the frame under the side sills.
The Friends of the C&TS have done an excellent job of restoring enough idler flats and pipe gons to make a credible display. This five car set has been used on several freight train specials since its completion a few years.
There were other flats and gons used for pipe as well, rebuilt from 39 and 40 foot standard gauge box and stock cars during the mid to late fifties. One of the rider gons on the C&TS is the ONLY remaining example of these in its original converted form (the other was "modified" into the other rider gon with its "pretty" vertical car-siding sides. (Don't get me started on this one
).
Anyway, the present rider gons on the D&S are from this same class of converted standard gauge cars that the Rio Grande re-built a second time in the Burnham shops (Denver) to use on their fledging Rio Grande Land endeavour that is the predecssor of today's D & S NG.
I know you only asked about the 30 foot pipe gons, but the 40 foot cars were found in the same trains and used for the same purpose just without the need for idler flats.
Long answer to an even longer story. Search the DPL library collection and you'll likely find some photos of the idler flats and pipegons in service.