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 taxi entrances, 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 12 tracks (even numbers from 8 to 30) on the south side of the station, each with a passenger platform on one side (served by two tracks), and a staff platform (also served by two tracks) on the other side, which connects into the station basement. Many support columns for the levels and buildings above are arrayed throughout the track level. At the south end of the platforms, still underground, among the support columns, the tracks come together in the station throat (many crossovers), reducing to seven tracks (numbered 8, 6, 5, 1, 2, 3, 4 from west to east) at Jackson Boulevard (MP 0.0), past the basement level of the Post Office on the west side of the tracks, where loading docks still remain alongside the westernmost track. The tracks emerge into daylight, reducing to "Four Main Tracks" (numbered AA3, AA1, AA2, and AA 4 from west to east), even though there are six physical parallel tracks at this point, at Harrison Street (MP 0.25), still with the Post Office on the west side, and the location of the erstwhile huge PRR freight house on the east side, with the Chicago River beyond it, where there is a signal bridge (six north-facing signals over the six parallel tracks) followed by a massive set of crossovers, including double-slip switches, at which the tracks of Amtrak's 14th Street Yard split away on the east side, and the leads to Metra's former CB&Q 14th Street Coach Yard split away on the west side (Polk Street, MP 0.41, Taylor Street, MP 0.57).
The Roosevelt Road concrete bridge passes above all of the tracks before the Metra yard spreads out on the west side, and has north-facing signals mounted on its north side at Roosevelt Road (MP 0.74). The CB&Q line's Two Main Tracks split away from the Chicago Union Station tracks at a cantilever signal bridge with south-facing signals over the westernmost two tracks, and begin to curve away to the west on the north leg of the Canal Street Wye, with the other tracks, now reduced to Two Main Tracks (AA1 and AA2) continuing due south, with south-facing signals over these tracks (13th Street, MP 0.85, 16th Street, MP 1.21, 18th Street, MP 1.40), passing beneath the viaduct (and through girder bridge, over the tracks heading south) that once carried the tracks of the B&O Chicago Terminal into Grand Central Station and the viaduct carrying the St. Charles Air Line eastward from Union Avenue and Halsted Street to its former junction with the Illinois Central main line at South Wye Junction.
The southward tracks pass the south leg of the Canal Street Wye at CP Lumber Street (MP 1.47), where there are crossovers adjacent to Amtrak's 18th Street Shops on the east side of the line, and extra tracks on the west side, the south leads from Amtrak's shops trail in on the east side, there is a road bridge overhead followed by a warehouse on the east side of the line that extends all the way to the bend of the river towards the South Branch Bridge.. There are south-facing signals south of the Amtrak leads, a grade crossing (West 20th Place), and north-facing signals (three heads) at each track, and the line crosses over the two track South Branch Bridge (MP 1.54), the lift bridge crossing the Chicago River to the end of Amtrak (Union Station) trackage at 21st Street (CR-IC Junction, MP 1.58), where there is a flat crossing (IC Crossing MP 1.64) with the Illinois Central trackage heading from16th Street Crossing on the St. Charles Air Line to its westward line starting at Bridgeport, with connecting track in the northwest quadrant, for trains heading (once) to the Santa Fe line to Joliet and (still) to the IC line to Joliet (the "Heritage Corridor"), and in the southeast quadrant (CR-C&WI Junction, MP 2.02). The CTA Orange Line bridges overhead just south of the flat crossing.
The westward tracks come together at Canal Street Wye, with signals on the south leg of the wye, and the tracks south out of the Metra yard curve around to join them just west of that location, with the South Canal Street road bridge overhead, just west of the wye, as the line comes along the south side of the St. Charles Air Line viaduct, descending to the west to join with the main line at Union Avenue (MP 1.7), directly beneath the overhead concrete bridge carrying I-90/94, where there are two sets of crossovers with a cantilever signal bridge, and Four Main Tracks begin, heading westward. South Union Avenue passes beneath immediately west of the Interstate Highway bridge, and there is a Metra station with island platforms serving pairs of tracks at Halsted Street (MP 1.8), with a signal bridge located above the platforms. The speed limit is 25 mph for passenger trains and 10 mph for freights east of MP 1.4, 35-15 thence to MP 1.7, and 45-15 west of MP 1.7.