The Green Diamond was a streamlined passenger train operated by the Illinois Central Railroad between Chicago, Illinois and St. Louis, Missouri. It operated from 1936 until 1968. It was the Illinois Central's first streamliner. Initially it operated with Illinois Central 121, the last of the 1930s fixed-consist articulated streamliners built in the United States. After 1947 it operated with more conventional streamlined equipment until its discontinuance. The name honored the "green diamond" in the Illinois Central's logo as well as the Diamond Special, the Illinois Central's oldest train on the Chicago-St. Louis run.