The Denver, South Park, and Pacific Railroad was a historic narrow gauge railway that operated in Colorado in the western United States in the late 19th century. The company operated as an independent railroad from its founding in 1872 until it was sold in foreclosure proceedings in 1889. Its lines later became part of the Colorado and Southern Railway. In the first half of the 20th century, nearly all the company’s original lines were dismantled or converted into standard gauge.

#51 was built in 1880 by Baldwin Locomotive Works as one of seven of this first 2-8-0 type on the road. When in 1885 engines were renumbered to fit in with Union Pacific system numbering, it became 191. It currently reside at the Colorado Railroad Museum in Golden, Colorado, on static display. #191 is the oldest remaining authentic Colorado locomotive in the state.