St. Charles Air Line (Halsted Street and 21st Street to South Wye Junction)

East of Halsted Street and west of the Union Avenue connection, on the former CB&Q, Amtrak trains stop and reverse, over the junction at Union Avenue, onto the St. Charles Air Line viaduct passing over the top of the lines coming south out of Chicago Union Station, with the former routing of the B&OCT line into Grand Central Terminal just to the north, crossing over the Chicago River on a bascule lifting bridge (with the permanently open B&OCT bridge of similar type just to its north), and descends to ground level, with a connector south to the Metra Rock Island District line, and a flat crossing (16th Street Crossing) over the Metra Rock Island District tracks coming south out of La Salle Street Station, to the junction at Clark Street.

From 21st Street (MP 2.6), the Illinois Central tracks that run along the northwest side of the route of the former C&WI tracks into Dearborn Station head northeast, past the former location of Santa Fe's 18th Street Passenger yard on the east side, with the CTA Orange Line's elevated tracks occupying part of that site, and climb gently towards a connection with the St. Charles Air Line, making a flat crossing with the southwest quadrant connector from the Union Avenue line and a flat crossing (also 16th Street Crossing) over the Metra Rock Island District tracks coming south out of La Salle Street Station, to the junction at Clark Street (MP 2.0), where the Union Avenue and 21st Street lines come together and continue east, past the location where they bridged over the C&WI tracks into Dearborn Station, and under the CTA elevated coming south from The Loop, to South Wye Junction (MP 1.2), where there was once a wye with the line coming south from (Illinois) Central Station and today's line turns south-southeast.

The speed limit is 10 mph across this entire single track line.