According to the Union Depot's website, the St. Paul Union Depot Co. was established in 1879 to consolidate service to passengers and comprised nine different railroads. Railroad tycoon James J. Hill oversaw the construction of the first passenger train terminal in St. Paul.

The first Union Depot was built in 1881, but after two fires in 1884 and 1913, the building was destroyed. A replacement was commissioned for the same location at Fourth and Sibley streets in Lowertown.

The St. Paul Union Depot Co. decided it needed a grand, spacious building with plenty of room for passengers and multiple rail lines to accommodate increasing demand.

Architect Charles Sumner Frost was tapped to design the new depot. Over his career, he designed more than 100 railroad buildings including the Great Northern Depot in Minneapolis. Work began on the new depot in 1917, although World War I and an economic turndown slowed progress.

The head house, which is still standing today, was opened to the public on April 3, 1920. The rest of the terminal would be built in stages from 1922-24, according to the Minnesota Historical Society.

St. Paul's Union Depot quickly became a major rail hub in the Midwest. According to the Minnesota Historical Society, it was the third-largest railway mail transfer station in the country behind New York and Chicago. In 1953, more than a quarter of all mail on all carriers was shipped through St. Paul.

Amenities for passengers included a dining room, barbershop, drug store, coffee shop and a 12-lane bowling alley.

During World War II, its massive skylights were blacked out with tar to protect soldiers using Union Depot as a staging area for deployment.

After the war, the number of trains passing through Union Depot swiftly declined.

The last passenger train, Burlington's Afternoon Zephyr, left from St. Paul Union Depot on April 30, 1971. The next day, all city-to-city passenger rail service was switched over to Amtrak, which operated out of Minneapolis.

Union Depot's concourse and waiting room were shut down.

The St. Paul Union Depot Co. and Minnesota Transfer Railway Co. still operated out of Union Depot until 1974, when they moved to the newly built Midway Station in St. Paul. Amtrak service eventually moved there, too, in 1978.

The U.S. Postal Service, which used to own the building across Sibley Street from the train deck, bought the concourse and waiting room in 1978. All the tracks and platforms were ripped out, and the area was converted for mail truck use.

The head house was added to the National Register of Historic Places in 1974, and it underwent a vast $243 million renovation from 2011-12 under the purview of the Ramsey County Regional Rail Authority.

St. Paul Union Depot's passenger rail days weren't over, though. In 2014, Amtrak's Empire Builder line resumed service to the historic station, and the Metro Transit Green Line started running on the north side of Union Depot the same year.