The St. Charles Air Line is a rail line in Chicago, Illinois, partially owned by the BNSF Railway, Union Pacific Railroad, and Canadian National Railway. It is currently used by the Canadian National Railway for freight trains and by Amtrak passenger trains. The line runs east from south of Union Station to a junction with Canadian National Railway at 16th Street interlocking (the CN line then continues towards the shore of Lake Michigan, where it turns south under McCormick Place, passing over and then paralleling the Metra Electric Line). History The line was chartered in 1852 as the Chicago, St. Charles and Mississippi Air Line Railroad, planned to run from Chicago west to the Mississippi River at Savanna via St. Charles. The Chicago depot would be at the northeast corner of Stewart Avenue and 16th Street. This line would compete with the Galena and Chicago Union Railroad, which thus opposed the project, and chartered the Dixon Air Line Railroad from St. Charles west to Dixon, Illinois. Eventually the St. Charles Air Line, an unincorporated jointly owned line, was formed as a reorganization of the project. It only built from the Illinois Central Railroad (also used by the Michigan Central Railroad) on Lake Michigan, near 14th Street, west along the original alignment to Western Avenue. From there a connection was built north to the Galena and Chicago Union Railroad, finished January 1, 1856. (The west end of the jointly owned line was, and still is, the west bank of the Chicago River.) On March 30, the G&CU and Chicago, Burlington and Quincy Railroad began using it to access the Illinois Central's Central Station. The planned alignment west of Western Avenue was later used by the Chicago and Northern Pacific Railroad, and piers in the Fox River at St. Charles had influenced predecessors of the Chicago Great Western Railway to build their line through that town. The Chicago, Burlington and Quincy Railroad also built a line into Chicago, intersecting the Air Line at Western Avenue. Eventually, the line came under equal control of the four companies that used it: the Illinois Central Railroad, Michigan Central Railroad, Chicago, Burlington and Quincy Railroad and Chicago and North Western Railway (successor to the Galena and Chicago Union Railroad). The Air Line's track was originally on ground level with numerous street crossings. In the late 1890s, work was begun to raise the line onto fill and