The D&RGW K-27 class are 3 foot narrow gauge, Mikado type, 2-8-2 steam railway locomotives built for the Denver and Rio Grande Railroad by the Baldwin Locomotive Works in 1903. They eventually became known by the nickname "Mudhens".
Fifteen locomotives were built, originally class 125, reclassified K-27 in 1924 when the Denver and Rio Grande became the Denver and Rio Grande Western Railroad. The K-27s were built as Vauclain compounds, with two cylinders on each side, expanding the steam once in the smaller cylinder and then a second time in the larger one. The extra maintenance costs of the two cylinders were greater than the fuel saving, so they were converted to simple expansion in 1907–1909. They were Rio Grande's last purchase of compound locomotives. They were built with their main structural frames outside the driving wheels, with the counterweights and rods attached outside the frames.
They had one peculiarity which arose from their outside frames and counterweights. In places where the D&RG's standard gauge system met the narrow gauge system, the railroad operated dual gauge trackage, with three rails, so that standard gauge equipment ran on the outer two rails and three foot gauge equipment ran on one of the outer rails and a third rail, inside the other two. Since the narrow gauge equipment was much lighter than the standard gauge, the inner rail was generally lighter and, therefore, not as tall as the standard gauge rails. In the case of the D&RGW, the difference was ⅞ inch (22 mm). Because the counterweights were outside the frames, they ended up directly over the standard gauge rail, with a clearance of only about ⅝ inch (16 mm). When the shop crews trued up the drivers periodically, they had to be very careful not to go too far.
They pulled freight, passenger and mixed trains on the D&RGW in and over the Colorado Rocky Mountains, traversing the entire length of the railroad. Many of them also spent time on the Rio Grande's subsidiary, the Rio Grande Southern.
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... The K-27s went through a variety of modifications during their 36 to 107+ years of service ... They ended up in three distinct groups, with many different details such as the location of the air tanks, whether or not they had a doghouse on the tender for the head brakeman, and so forth ...