The Monarch Branch was a branch of the Denver & Rio Grande Western which left the narrow gauge transcontinental route at Poncha Junction a few miles west of Salida, Colorado. The narrow gauge branch built in the 1880s served Colorado Fuel & Iron limestone quarry at Monarch, Colorado. The upper part of the Branch was on a 4.5% grade and included a double switch back to gain elevation to over 10,000 ft (3,000 m). In 1956 with the decline of narrow gauge operations out of Salida the line was converted to standard gauge. In 1982 Colorado fuel & Iron closed its blast furnaces at Pueblo and the branch was abandoned.
While the branch was operating it saw a daily train. In later narrow gauge days it the train was worked by K-36s or K-37s. The stone was loaded into narrow gauge wooden gondola cars the stone would be transferred to standard gauge gondola cars via a purpose built rotating 'barrel' at Salida. Once the branch was converted to standard gauge it was worked with 4 axle GP9s and in later days GP30s, GP35s and GP40s. 6 axle SD9s were tested on the branch but derailed due to the tight curves. Diesels assigned to the branch were given modified dynamic brakes to cope with the 4.5% grades.