O.K., here it is right from the book.
The grandaddy of the D-500 was the tender of the DRGW 964. It was used to build idler car 010798 in Jan. 1940. It was equipped with three-way couplers and used at Leadville Colorado. In 1950 the car was sold to the San Luis Valley Southern where it was used for the frame of the D-500.
There was actually two D-500's. The first one used rubber tires between the trucks. It didn't work out, so it was rebuilt into the present D-500.
This came from the book Colorado's Loneliest Railroad by Bob Griswold