I must respectfully disagree with something which has been stated here, and also in a previous thread which I started in 2008 about the old snowjet conversions. To quote Chief Troll from the older thread, "Those were not really conversions. Cleveland Technical Center, the NYC research subsidiary at Collinwood, mounted the jet engines and duct work on flat cars, and mounted old wood caboose car bodies, complete with cupolas, on the other end of the flats to house the operator." While it may well be true some, or even most, were made by putting the jet and part of a caboose body on a flat car, I strongly believe at least one or two were made right on the caboose itself. There are several photos around of those old units, most of fairly poor quality. But some show enough detail to support my claim.
* In the photo below, notice the short length of the machine - it appears shorter than even a 40 foot flat would be.
* Note the spacing of the trucks. Far too close together to be a flat car - and just about exactly how the old cabooses were spaced
* While the end closer to the camera has had the end squared off and stirrups and grab irons applied (a fairly easy change), note the far end still has the original wooden caboose steps and end handrails. It seems unlikely to me that - if things were being transplanted to a flat car - they would go to the trouble of cutting out the end of the flat car and then installing the wooden steps and end platform off the caboose. It would have been far more expedient to fabricate and weld steel steps.
* Note also the underside of the car in the photo, where you can clearly truss rods. Compare those truss rods with a 19000 series caboose and they are identical. By the time those blowers were made, truss rod freight cars were pretty scarce, if not gone altogether.
A caboose photo taken at about the same angle for comparison
Again, I don't doubt that flat cars were used at some point. But I believe that at least one was made directly on the caboose frame itself.