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 replace the grade crossings with overpasses.
As originally constructed, the east end of the Air Line connected with the IC with connections that curved to the north to serve both Central Station and the yards and warehouses through downtown up to the Chicago River. There was a shorter, steeper ramp down for passenger trains to get into Central Station while a longer, less severe incline was used by freight trains. In 1968, a southward-facing connection was built, generally known as the South Leg because together with the original lines a wye was formed which was occasionally used to turn passenger trains around. The South Leg enables trains using the Air Line to travel directly to and from the south. After the coming of Amtrak in May 1971, the remaining passenger trains were gradually shifted from Central Station to Union Station and the northern connections were removed. This coincided with the gradual elimination of yards to the north (this area is the Illinois Center office, hotel, and retail complex today). Central Station closed in 1972 and was razed in 1974. Now the South Leg is the only connection from the IC mainline to the Air Line.
In the 1980s and 1990s, Chicago city planners wanted to tear down the St. Charles Air Line as part of initiatives to redevelop Chicago's Near South Side neighborhood.[1] Chicago has since reversed its position, adopting plans to run more transit and intercity passenger trains on the St. Charles Air Line.[2]: 6
As of 2011[update], the St. Charles Air Line hosted six Amtrak trains and about four to five CN freight trains per day.[3]
Amtrak's City of New Orleans, Illini, and Saluki utilize this line with a time-consuming switchback into Union Station from its west end. The Chicago Region Environmental and Transportation Efficiency Program (CREATE) is in the preliminary design phase for the Grand Crossing Project. This project will reroute the passenger trains from St. Charles Air Line to Norfolk Southern's Chicago Line in the Greater Grand Crossing neighborhood in Chicago.[4] The CN also uses it as a freight connection. As of October 2021, a large project was nearing completion to replace a bridge passing over the future Wells Street Extension, and rebuild retaining walls on both lines between the 16th St. interlocking and the Chicago River. However, this section of the line has been inactive since May 2020 when the adjacent St. Charles Air Line Bascule Bridge crossing the river was elevated due to work on the west side of the river.
The Canadian National Railway's employee timetable dated July 2011 states: "St. Charles Air Line is a connection between Metra 16th St.
Interlocking and BNSF Union Ave. BNSF and UP jointly own the line between Union
Ave. and a point 70 feet west of the Bascule Bridge over the South Branch of the
Chicago River. CN owns the line from that point to 16th St. ...
Both tracks are designated Non-Main Track, CN Rule 520 applies, and is also
designated East-West. The north track is Track 1, and the south track is Track 2.
... The bridge is controlled by CSX Bridge Tender."
In 2019, CN begin a track realignment program to the east of the St. Charles Air Line Bridge. The realignment changed a double-track section to single track in an east-west corridor running north of 16th St., which originally consisted of four elevated lines laid over two parallel series of viaducts that cross five streets and two alleys.[5] Replacement of the viaduct bridges was finished in 2020, and the entire project was completed in 2021 after the placement of about 4,000 feet (1,200 m) of track.[6]
In 2022, Amtrak began seeking grants to build a direct connection between the St. Charles Air Line and Union Station to avoid the switchback maneuver. Amtrak might also try to lease or buy the St. Charles Air Line to relieve congestion for its passenger trains.[7] As part of the congestion relief effort, Amtrak wanted to reroute its Michigan and East Coast trains onto the St. Charles Air Line after the Union Station direct link was made,[8] and it applied for funding to restore the roughly 1 mile (1.6 km) double-track alignment that CN just removed.[9] However, the funding requests were largely rejected in the December 2023 funding round of the federal government's Corridor Identification and Development Program.[10]