Chicago Union Station is an intercity and commuter rail terminal located in the West Loop neighborhood of the Near West Side of Chicago. Amtrak's flagship station in the Midwest, Union Station is the terminus of eight national long-distance routes and eight regional corridor routes. Six Metra commuter lines also terminate here.

Union Station is just west of the Chicago River between West Adams Street and West Jackson Boulevard, adjacent to the Chicago Loop. Including approach and storage tracks, it covers about nine and a half city blocks (mostly underground, beneath streets and skyscrapers, some built with the earliest usage of railway air rights).

The present station opened in 1925, replacing an earlier union station on this site built in 1881. The station is the fourth-busiest rail station in the United States, after Pennsylvania Station, Grand Central Terminal, and Jamaica station in New York City, and the busiest outside of the Northeast Corridor. It handles about 140,000 passengers on an average weekday (including 10,000 Amtrak passengers). It has Bedford limestone Beaux-Arts facades, and an interior with massive Corinthian columns, marble floors, and a Great Hall, highlighted by brass lamps.

The station connects to multiple transit authorities including the Chicago Transit Authority bus and Chicago L lines, Metra, Pace, Greyhound, and more either within the station or within walking distance.

Chicago Union Station is named a union station, like many train stations across the United States that were shared by several railroad companies. The station is the third union station to occupy the site between West Adams Street and West Jackson Boulevard. The station is known by the acronym CUS, as well as by its Amtrak station code CHI.

Chicago Union Station is situated in the West Loop Gate neighborhood of the Near West Side of Chicago, just west of Chicago's Loop. The station's underground concourse and train sheds abut the Chicago River; passageways extend west beneath Canal Street to the main station building, one block over.

Chicago Amtrak services
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Amtrak services within the Metra service area
Empire Builder to Seattle or Portland Hiawatha to Milwaukee Borealis to St. Paul Metra MD-N Glenview MD-N Metra UP-NW Metra MD-W NCS Metra UP-W Metra to Ogilvie Transportation Center Union Station South Shore Line & Metra ME to Millennium Station switchback for I&S and CoNO Homewood ME Illini and Saluki to Carbondale City of New Orleans to New Orleans Summit HC Metra RI to LaSalle Street Joliet HC RI Lincoln Service to St. Louis Texas Eagle to San Antonio La Grange Road BNSF Naperville BNSF California Zephyr to Emeryville Southwest Chief to Los Angeles Illinois Zephyr and Carl Sandburg to Quincy Not all lines and junctions shown
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Unlike many major American intercity and commuter rail hubs, Union Station does not have any direct connection to local rapid transit service. However, two Chicago "L" stations are within walking distance of Union Station.

• Clinton (two blocks south of the station): Blue
• Quincy (three blocks east of the station, on The Loop): Brown, Orange, Pink, and Purple

Metra's other three downtown terminals – the Ogilvie Transportation Center, LaSalle Street Station and Millennium Station – are all within walking distance of Union Station. Passengers connecting from Ogilvie can access Union Station's north platforms on the opposite side of Madison Street.

Numerous CTA bus routes stop directly at Union Station:

• Union Station Transit Center: 1, 28, 121, 124, 128, 151, 156
• West side of Clinton, north side of block: 125, 130, 192
• West side of Clinton, south side of block: 7, 60, 157
• Southwest corner of Clinton/Jackson: 126, 754, 755 (Pace)

Union Station Transit Center is located adjacent to Union Station's parking garage. The bus station opened in 2016, on land formerly used for a surface parking lot. It features an elevator and stairway to the Amtrak underground pedestrian tunnel, allowing commuters to pass between Union Station and the bus staging area without crossing at street level.

Union Station has a counter operated by the Greyhound intercity bus company. Tickets are available for purchase, and some Greyhound and Megabus buses pick up passengers on South Canal Street, on the east side of the station building. The full-service Greyhound station is four blocks southwest of Union Station.

Union Station was served by lines in all directions even before Penn Central and Amtrak consolidated the downtown terminals. The station served as a terminal for the following railroads:

• Chicago and Alton Railroad – only a tenant, later part of the Gulf, Mobile and Ohio Railroad
• Chicago, Burlington and Quincy Railroad (Burlington Route)
• Chicago, Milwaukee, St. Paul and Pacific Railroad (Milwaukee Road) – The station housed its corporate offices from 1924 to 1986
• Pittsburgh, Fort Wayne and Chicago Railway (PRR)
• Pittsburgh, Cincinnati, Chicago and St. Louis Railroad (Panhandle) (moved to use the PFW&C approach after April 23, 1917)
• Penn Central Transportation Company (former services of the New York Central Railroad and Michigan Central Railroad) (moved from LaSalle Street Station October 27, 1968)
• Amtrak (began May 1, 1971, moved from Dearborn Station May 2, 1971 and Central Station March 6, 1972 (Floridian moved January 23, 1972); Amtrak's Calumet and Indiana Connection commuter trains also ran into Union Station)

Burlington Route	Gulf, Mobile and Ohio	Milwaukee Road		Pennsylvania		Great Northern	Northern Pacific
California Zephyr Denver Zephyr Twin Cities Zephyrs American Royal Zephyr Kansas City Zephyr Western Star	Alton Limited Abraham Lincoln Ann Rutledge Midnight Special	Twin Cities Hiawatha Midwest Hiawatha Pioneer Limited Olympian Columbian Olympian Hiawatha	City of San Francisco City of Los Angeles City of Portland Challenger Sioux The Arrow	The Admiral Broadway Limited The General Trail Blazer Cincinnati Daylight Express	The Fort Pitt Pennsylvania Limited Pennsylvanian Manhattan Limited South Wind Gotham Limited	Empire Builder Western Star	North Coast Limited Mainstreeter

Some of these trains and their names have survived to present Amtrak services, such as the Empire Builder, the California Zephyr, and the Hiawatha.

The name Ann Rutledge was used by Amtrak as a Chicago-St. Louis-Kansas City route until a name and service consolidation in 2009. There is also a Lincoln Service that operates in Illinois.

Located west of Canal Street, Union Station's station building occupies an entire city block. At its center is the Great Hall, the main waiting room. Arrayed around the Great Hall are numerous smaller spaces containing restaurants and services, and a wide passageway leading to the concourse. Above the main floor are several floors of office space, currently used by Amtrak. Original plans called for many more floors of offices, forming a skyscraper above the Great Hall. This was never completed, although the plan has been revived in recent years.

The Burlington Room is an event space at the northwest of the Great Hall. The room features large columns, chandeliers, four French block murals of landscape scenery, and an original mirror. The space, initially a women's lounge, was restored in November 2016, after years of damage and neglect. For event uses, the space has color-changing lights and an audiovisual system.

The headhouse includes a space formerly used as a Fred Harvey restaurant. After a large fire in 1980, the space was damaged, windows on Clinton Street were destroyed, and the space was left vacant since then. In 2018, Amtrak announced plans to redevelop the space into a multi-level food hall, using funds from the sale of its parking garage. A new entrance and canopy would be installed on Clinton Street, and new windows would replace the bricked-up windows. The food hall was planned to open in the summer of 2020.

The headhouse also includes a Metropolitan Lounge, one of seven Amtrak offers in its stations. The lounge operates like an airport lounge, accessible to business- and first-class passengers, as well as other high-price ticketed passengers. The lounge reopened in June 2016, moving from the concourse to the headhouse. It has two stories and 13,500 square feet (1,250 m), double the space of the previous lounge. It features different seating areas intended for businesspeople, families and children, and people using phones or tablets. The space has bathrooms with showers, and an elevator.

Union Station is laid out with a double stub-end configuration, with 10 tracks coming into the station from the north and 14 from the south. Unlike most of Amtrak's major stations, every train calling at Union Station either originates or terminates there; all passengers traveling through Chicago must change trains to reach their final destination. There are two through tracks to allow out-of-service equipment moves between the north and south side, including one with a platform to allow extra long trains to board. Between the north and south sides of the station is a passenger concourse. Passengers can walk through the concourse to get from any platform to any other without stairs or elevators. Odd-numbered platforms (1–19) are on the north half of the station, and even-numbered platforms (2–30) on the south half. The north tracks are used by Amtrak for the Hiawatha, the Borealis, and the Empire Builder, and by Metra for the Milwaukee District West, Milwaukee District North, and North Central Service routes. The south tracks are used for all other Amtrak services, as well as by Metra for the BNSF, Heritage Corridor and SouthWest Services. Two station management structures (known as glasshouses), one on each side of the terminal, monitor train-to-track assignments and the flow of traffic in and out of the station. Actual oversight and control of switching and signalling is accomplished by two "train director" positions, one for each side of the station, located in the Amtrak control center in the station's headhouse.

Numerous entrances provide access to Union Station's underground platform level. The main entrance is on Canal Street opposite the headhouse, but passengers can also reach the platforms directly from the headhouse via an underground passageway. Two secondary entrances are located in Riverside Plaza near the Jackson Boulevard and Adams Street bridges. On Madison Street, across the street, and one block east from Ogilvie Transportation Center, are a set of entrances to the north platforms.

Union Station was designed by D. H. Burnham & Company (known for its lead architect Daniel Burnham, who died before construction began). The successor firm of Graham, Anderson, Probst and White completed the work. The terminal was among the first to anticipate automobile traffic; it was first designed in 1909, one year after the Model T entered production. It was designed with ticket offices, concourses, platforms, waiting and baggage rooms, and shops, all on a single level, meant to be easy to navigate. At opening, the terminal also housed a hospital, chapel, and jail cell.

The main building, a square Neoclassical structure, takes up one city block. Its architectural style contrasts with modern glass-faced buildings around it. The station has wide porticos and large colonnades on its exterior. The street-level entrances utilize Indiana limestone.

The station originally featured a large Beaux-Arts concourse building along the river, made with marble, glass, and iron. Massive steel arches held up the vaulted roof, and several stairways led passengers down to the platforms. The concourse was demolished in 1969 and replaced with an office tower.

At the building's center is the Great Hall, a 110-foot (34 m)-high atrium capped by a large barrel-vaulted skylight. The 24,000-square-foot (2,200 m) room has connecting lobbies, staircases, and balconies. Enormous wooden benches are arranged in the room for travelers to wait for connections, and two specially-designed underground taxicab drives were built to protect travelers from the weather. The room's columns are of textured Roman travertine, with leafy golden Corinthian capitals. The ceilings and insets are coffered, with decorative rosettes.

Two statues by Henry Hering —Night and Day — look down on passengers, symbolizing the 24-hour operation of the railroads. The statue Night holds an owl, while Day holds a rooster.

The Chicago Union Station Power House is a decommissioned coal-fire power plant that provided power to Union Station and its surrounding infrastructure. Located on the Chicago River, north of Roosevelt Road, it was designed in the Art Moderne style by Graham, Anderson, Probst and White in 1931. The power plant was decommissioned in 2011. It was included in Preservation Chicago's 7 Most Endangered list in 2017 and 2020, as Amtrak has plans to demolish the building.

The same architecture firm that designed Union Station also designed the Old Chicago Main Post Office, a post office atop the station's southern tracks. The post office, opened four years before Union Station, utilized the rail system, funneling mail to and from the trains below. An expansion in 1932 made the structure the world's largest post office.

The large amount of land above the tracks and platforms has tempted property owners and developers. Possibly inspired by Terminal City, a development built atop New York's Grand Central Terminal's train shed, Chicago moved to develop the air rights above Union Station's tracks. The first building to be built was that of the Chicago Daily News in 1929. Designed in the Art Deco style, it was the first structure to add a public promenade along the river, which would be named Riverside Plaza. Soon after, in 1932, the new Chicago Main Post Office opened. Also in the Art Deco style, it is a gigantic structure that occupies two full city blocks. The Great Depression and subsequent World War II halted development, but in the 1960s, work began on Gateway Center, a Modernist complex of five buildings. Only the first four were built, and construction lasted into the 1980s through several economic cycles.

In 1990 the Morton International Building opened. Now named for Boeing, it is the tallest building yet to be constructed over the tracks. With the construction of River Point beginning in 2013 and 150 North Riverside beginning in 2014, the entire length of the train shed and tracks from Union Station north to Fulton Street and south to Polk Street is enclosed by overhead development.

Chicago Union Station's train shed, covered by buildings built above the tracks, helped lead locomotives to funnel significant soot and smoke in and around the station. This was unlike Grand Central Terminal, which has only allowed electric trains into its trainshed since opening.

The current Union Station is the second by that name built in Chicago, and possibly the third rail station to occupy the site. The need for a single, centralized station was an important political topic in 19th and 20th-century Chicago, as various competing railroads had built a series of terminal stations. The numerous stations and associated railyards and tracks surrounded the city's central business district, the Loop, and threatened its expansion. The various stations also made travel difficult for through-travelers, many of whom had to make inconvenient transfers often slowed by street traffic from one station to another through the Loop. Union Station was part of architect Daniel Burnham's city-wide Plan of Chicago in 1909.

On December 25, 1858, the Pittsburgh, Fort Wayne and Chicago Railroad opened as far as Van Buren Street in Chicago. It built the first station at what would eventually become today's Union Station on the west bank of the Chicago River.

The railroad built a permanent depot at the corner of Canal and Madison streets in 1861.

On April 7, 1874, five railroads agreed to build and share a union station just north of the original Pittsburgh, Fort Wayne, and Chicago Railroad station site at Van Buren Street. These railroads were:

• Pennsylvania Company (a subsidiary of the Pennsylvania Railroad)
• Chicago, Burlington and Quincy Railroad (Burlington Route)
• Michigan Central Railroad
• Chicago and Alton Railroad
• Chicago, Milwaukee and St. Paul Railway (The Milwaukee Road)

The Michigan Central, which had previously been using the Illinois Central Railroad's Great Central Station, soon decided to back out of the agreement, and continued to use the Illinois Central Depot. The Chicago and North Western Railway, not part of the original agreement, considered switching to the new station from its Wells Street Station but deferred instead. In 1911 it built the Chicago and North Western Passenger Terminal for its operations.

The remaining four original companies used the station when it opened in 1881. The headhouse of the Union Depot, a narrow building, fronted onto Canal Street and stretched from Madison Street to Adams Street. Tracks led into the station from the south, and platforms occupied a strip of land between the back of the headhouse and the bank of the Chicago River. South of the station, Adams, Jackson, and Van Buren Streets rose over the tracks and the river on bridges. The station, along with its successor, was effectively two back-to-back stub-end terminals. Virtually all trains arriving would terminate there, and passengers traveling further would need to change trains.

Growth in passenger traffic, as well as a civic push to consolidate numerous railroad terminals, led to a proposal for an enlarged Union Station on the same site. The second Union Station would be built by the Chicago Union Station Company. This was a new company formed by all the railroads that had used the first station, save for the Chicago and Alton, which became a tenant in the new station. The Pennsylvania Railroad, then the U.S.'s largest railroad company, planned and directed the project.

The architectural firm was D. H. Burnham & Company (known for its lead architect Daniel Burnham, who died before construction began). The successor firm of Graham, Anderson, Probst and White completed the work. Work began on the massive project in 1913, and required purchasing adjacent properties and moving freight facilities. Construction stalled during World War I, and resumed in 1919. The station finally opened on May 16, 1925, twelve years after construction began; some viaduct work continued into 1927. The construction cost, funded by the railroad companies involved, was projected to be $65 million, but ended up costing $75 million. Construction was delayed several times by World War I, labor shortages and strikes. The construction of the station also involved the demolition and relocation of some previously existing buildings such as the Butler Brothers Warehouse along the Chicago River. It is one of about a dozen monumental Beaux-Arts railroad stations that were among the most complicated architectural programs of the era called the "American Renaissance", combining traditional architecture with engineering technology, circulation patterning and urban planning. Union Station was hailed as an outstanding achievement in railroad facility planning at the time.

During World War II, Union Station was at its busiest, handling as many as 300 trains and 100,000 passengers daily, many of them soldiers. Illustrator Norman Rockwell captured this era with his cover painting for a December 1944 issue of The Saturday Evening Post, depicting the station jammed with Christmas travelers. After the war, the growth of highway construction and private ownership of automobiles caused a severe decline in American passenger-rail ridership, including at Union Station.