The D500 is a chain-drive diesel-mechanical locomotive built in Mesita, CO, 25 miles south of Blanca, at the workshops of the Colorado Aggregate Co. during 1954-56. It was designed and built by Channel Wilcox of Denver, CO and George Oringdulph of Mesita, CO to replace the SSLV's aging ex-D&RGW steam locomotives. A second-hand diesel was beyond the railroad's means so it decided to build its own. Available was former D&RGW idler car No.010798, which was the underframe and wheelsets (trucks) of a steam locomotive tender from which the coal bunker and water tank had been removed, and castings, each having one standard gauge and two narrow gauge coupler pockets, installed at both ends. Thus the D&RGW could use it to move both standard and narrow gauge cars in a single train over dual gauge track between Alamosa, CO and Antonito, CO. The rear coupler casting is missing but the front one remains in place.

The SSLV had no need to move narrow gauge cars but could use the idler as a basis for its project. An International Harvester UD-24 diesel, designed for bulldozers and IH's most powerful engine at the time, was installed driving, through a Caterpillar hydraulic transmission, the power axle from a Euclid truck. All were installed above the frame and enclosed in a new body and cab to become the D500 as here displayed. The Euclid axle connects, via sprockets and roller chains, with two transverse shafts, one above each of the two trucks. Each shaft is similarly connected using sprockets and chains with one axle of each truck. The other axle in each truck is connected by another set of sprockets and chains, so each axle is powered. The headlights and horn came from a road freight vehicle and the red running lights from a school bus. The “F”s on the black side ballast bins were required by Federal regulations to indicate the front end. There are two exhaust stacks because the UD-24 engine has two separate 3-cylinder heads, each with its own exhaust. Incredibly, it all worked!

After the D500 proved its ability to successfully operate from Blanca, CO to Jaroso, CO the SSLV, in 1957, retired its remaining operable steam locomotive. It, as the last D&RGW standard gauge survivor, is displayed at the Colorado RR Museum, Golden CO as D&RG No.583, its original number. The D500 last operated in the early 1990's, then sat on an isolated track section south of Blanca for another 30 years, seemingly abandoned but not unloved or forgotten by the people of Blanca, the San Luis Valley and rail fans across the Country and indeed further abroad, all anxious as to its fate. And so in 2024 it was donated to the Town of Blanca by the Colorado Pacific Rio Grande Railroad and moved to this location under the first roof it has ever had. This unique locomotive, product of San Luis Valley ingenuity and tenacity, is believed to be the only locomotive ever designed and built in Colorado.