Chicago Union Station was created in 1925 to be the joint station in Chicago of the Pennsylvania Railroad (PRR), Alton, Chicago, Burlington & Quincy (CB&Q), and Chicago, Milwaukee, St. Paul & Pacific (Milwaukee Road), as it was to become. In 2006, it is the sole remaining inter-city passenger station in Chicago, serving Amtrak trains, as well as serving Metra trains to Orland Park (NS), Joliet (IC "Heritage Line"), Aurora (BNSF), Elgin, and Fox Lake (both Milwaukee).
The station has two sets of stub-end platforms, one facing north and one facing south, with two through platforms on the east side and some additional through tracks on the east side, adjacent to the Chicago River. In between the ends of the north platforms and the south platforms is an area with waiting rooms, baggage rooms, and ticket counters, as well as former taxi entrances (closed after 9/11/2001), all at the same level as the platforms, above which is a mezzanine level with short-order restaurants, news stands and convenience stores, all of which are below ground level. Escalators and stairways carry people between the levels and to/from ground level. There is a passageway west, under the street, to the station's Great Hall on the west side of Canal Street, from which there are also stairways to the street level. The original headhouse between Canal Street and the Chicago River was replaced by an office building in the 1960s.
There are five platforms on the north side of the station (serving ten tracks numbered 1-19, odd numbers only, with service platforms between each pair of tracks, alternating with the passenger platforms), which have north entrances, directly down to each platform, on the south side of Madison Street, just west of the Chicago River (where the tracks are underground, under street bridges, or under air-rights buildings as far as Randolph Street), reducing to tracks numbered 2, 1, 3, 5 from the west side, which at Randolph Street (MP 0.36, where there is a street bridge overhead just outside the underground segment) then become tracks 3, 2, A, 1 from the west side and then tracks 3, 2, 1 at Lake Street (MP 0.45, where the tower on the northeast side, unused since February 9, 2006, was demolished on January 15, 2013), as the tracks run north along the west side of the Chicago River, turn north-northwest, pass beneath the steelwork trestle of the CTA Lake Street line heading west and over a grade crossing with a north-south street, then turn west and pass under the through girder bridge carrying the trackage in the throat of Northwestern Station (the Metra Madison Street Station), on a route built by the Pennsylvania Railroad to serve its trains from Cincinnati, Louisville, Indianapolis, and points between those locations and Chicago.
In the 2000s, some of the PRR trackage has been removed, and further west, other former PRR trackage lies along the south side of the Metra (former Milwaukee Road) trackage. The line is ABS as far as Canal Street (MP 0.57), ABS Rule 251 (current-of-traffic) as far as Green Street (MP 1.06), and under the control of Morgan Street (MP 1.25) interlocking to Racine Avenue (MP 1.5). Along the way, the line passes a grade crossing over Clinton Street, a north-south road bridge overhead, a grade crossing with a north-south street, a north-south road bridge overhead, a grade crossing with a north-south street, crossovers, three grade crossings with north-south streets, a signal bridge, extra track on the south side (beyond the existing three or four), a grade crossing with a north-south street, and two bridges over north-south streets.
Former C&NW trackage on its Geneva line curves westward from the station throat and descends along the north side of the trackage from Union Station, two or three tracks wide (and used to be more, on the north side), at Noble Street. The ex-Milwaukee Road line is again ABS Rule 251 (current-of-traffic) to Leavitt Street (MP 2.73), past a disused CTA through truss bridge overhead, and a signal bridge overhead, and then under the control of the Western Avenue interlocking to the eponymous location. At Western Avenue (Tower A-2, still standing in the northeast 'quadrant', MP 2.90), the three ex-Milwaukee Road tracks swing west-northwest and cross the ex-C&NW tracks at grade (double-slip switches) into the Metra Western Avenue station (MP 2.97), which has island and east-side platforms serving the three tracks here.