St. Louis Gateway Rail: The 1970s - Hardcoverby Lesley Barker (Author) Though the city of St. Louis is located on the Missouri side of the Mississippi River, for the railroads, the St. Louis Gateway extends into Illinois, north and south along both sides of the river. Two factors conspired against St. Louis's aspiration to become the preeminent rail center of the 19th century American Midwest: there was no bridge across the Mississippi, and Missouri's loyalty to the Union during the Civil War