The St. Charles Air Line is an interconnection between the BNSF, UP and
CN railroads that runs through the South Loop. It may be most notable as
the aerial route taken by Amtrak trains from the south and east as they
enter into Union Station.
A twin single-leaf Strauss Trunnion bascule railroad bridges carries the Air Line
over the South Branch of the Chicago River. The 186-foot northern span was built in 1930 for the Baltimore
and Ohio Railroad to lead to their (now demolished) Chicago terminal. Since it
no longer leads anywhere, it is locked open. The southern St. Charles Air Line
Bridge is a 220-foot span was originally built in 1919 but was shortened and
re-erected in 1930 when the South Branch was straightened.