Duluth, Missabe, and Northern Railway Company Map of the Mesabi Range

The Duluth and Iron Range Railroad Company and the Duluth, Missabe and Northern, both owned by United States Steel Corporation, were operated as one system from 1901 to 1937, although they were not formally merged. The DM&N leased the properties of the D&IR in early 1930. On July 1, 1937 the DM&N and the Spirit Lake Transfer Railway Company consolidated to form the Duluth, Missabe and Iron Range Railway Company (DM&IR). In March 1938 the property and assets of the D&IR and the Interstate Transfer Railway Company were transferred to the Duluth, Missabe and Iron Range. Interstate Transfer was subsequently dissolved on June 28, 1938, and the D&IR was dissolved July 7, 1938. The surviving company, The Duluth, Missabe and Iron Range, had two separate operating divisions: The Missabe Division, operating the former DM&N trackage on the western portion of the system, and the Iron Range Division, operating the former D&IR trackage on the eastern portion of the system.

Soft high-grade iron ore began to run out on the Vermilion and Missabe iron ranges in the mid-1950s, and at that time development of northeastern Minnesota's taconite industry began in earnest. With the passage of the Taconite Amendment in 1963 the construction of taconite plants along the DM&IR proceeded with the Eveleth Taconite Company's plant at Forbes, Minnesota coming into operation around 1965 and United States Steel Corporation's large Minntac taconite pellet plant at Mountain Iron beginning operation in October 1967.

In 1988 the DM&IR, a wholly-owned subsidiary of United States Steel Corporation, was sold to Transtar Inc., Monroeville, Pennsylvania. Transtar was a holding company owned in part by the Blackstone Group and USX Corporation (formerly known as United States Steel Corporation). In 2001 the railroad became part of Great Lakes Transportation, a privately-held transportation holding company with principal operations in railroad freight and transportation, Great Lakes shipping, and inland river barging.

In 2002 the Duluth, Missabe and Iron Range Railway was said to be the largest iron ore handling railroad in North America, with 212 miles of track. Its primary mission was to move ore from Minnesota's Missabe Range taconite plants to DM&IR dock facilities at Duluth and Two Harbors, or to connecting railroads at Superior, Wisconsin.