Colorado Fuel & Iron was the major steel producer in Colorado and a significant customer of the Rio Grande. Their mill in Pueblo took in natural resources collected from all over the D&RG system and shipped out final products.
The companies roots trace back to the Colorado Coal & Iron Company, founded in 1880 from the merger of three Denver & Rio Grande subsidiaries - the Colorado Coal and Steel Works Company, the Central Colorado Improvement Company, and the Southern Colorado Coal and Town Company.1 CC&I merged with the Colorado Fuel Company? in 1892 to form Colorado Fuel & Iron.
The blast furnaces were turned off in 1982, on account of the collapse of the American steel market. From then on, the Pueblo mill never processed raw ore again, only operating electric arc furnaces for remelting scrap. This change removed any need for limestone from Monarch, and as such the Monarch Branch was abandoned.
In 1993, the financially struggling CF&I was sold to Oregon Steel Mills. OSM, in turn, was sold to Russian steelmaker Evraz Group S.A. in January 2007. The Pueblo mill still produces
1 Wikipedia: Colorado Coal and Iron Company