The D500 was a chain-drive diesel-mechanical locomotive built in Mesita, Colorado, during 1954-56 at the workshops of the Colorado Aggregate Co., 25 miles south of Blanca, to replace the Southern San Luis Valley Railroad's aging ex-D&RGW steam locomotives. Designed and built by Channel Wilcox of Denver and George Oringdulph of Mesita because the railroad could not afford a second-hand diesel, it used former D&RGW idler car No. 010798 as its foundation. That idler, originally the underframe and wheelsets of a steam locomotive tender with the coal bunker and water tank removed, had coupler castings at both ends that allowed the D&RGW to move both standard and narrow gauge cars in a single train over dual gauge track between Alamosa and Antonito; the rear casting is now missing, but the front one remains. The Southern San Luis Valley had no need to move narrow gauge cars, but it could use the idler for its project. An International Harvester UD-24 diesel engine, designed for bulldozers and then the company's most powerful engine, was installed above the frame and drove, through a Caterpillar hydraulic transmission, the power axle from a Euclid truck, all enclosed within a new body and cab. Through sprockets, roller chains, and transverse shafts, every axle was powered. The headlights and horn came from a road freight vehicle, the red running lights from a school bus, the black side ballast bins carried federally required “F” markings for the front end, and the engine had two exhaust stacks because its two separate three-cylinder heads each had its own exhaust. The locomotive proved successful on operations from Blanca to Jaroso, and in 1957 the railroad retired its remaining operable steam locomotive. The idler car from which it had been built, the last D&RGW standard gauge survivor, is displayed at the Colorado Railroad Museum in Golden as D&RG No. 583, its original number. The D500 last operated in the early 1990s, then stood for about 30 years on an isolated track section south of Blanca before being donated in 2024 by the Colorado Pacific Rio Grande Railroad to the Town of Blanca and moved under the first roof it had ever had. It is believed to be the only locomotive ever designed and built in Colorado.