During World War II, the GM&O trimmed some branches and generally tightened up the organization. In 1947, the GM&O merged with the Alton, extending its lines west to Kansas City and north to Chicago and Springfield, Illinois, making the GM&O a Great Lakes-to-Gulf railroad.
The Gulf Mobile & Ohio was one of the first major U.S. railroads to dieselize completely, its last steam operations occurring on October 7, 1949. GM&O operated the 'Rebel' trains, the first streamliners in the south, between St. Louis and New Orleans, as well as the 'Gulf Coast Rebel' between St. Louis and Mobile. These lasted until 1954 and 1958 respectively.
At its peak, the GM&O operated 258 locomotives over 2,734 miles of track in the midwest and south. Some of its most famous name trains were the Ann Rutledge, the Alton Limited, and the Abraham Lincoln.
The colorful red and maroon GM&O colors lasted until its merger with the Illinois Central in 1972.