Union Station was the name of a transportation facility located on Earth. It was known for having a large volume of vehicles constantly arriving and departing.
In 2151 aboard Enterprise NX-01, Hoshi Sato claimed that the launch bay was like Union Station with shuttlecraft coming and going all the time. She did so after Sopek, aboard a Vulcan vessel, wanted to know where a shuttlepod was going. (ENT: "Shadows of P'Jem")
The name "Union Station" is used by over a hundred past and present railroad stations in over thirty states and the District of Columbia, as well as Toronto, Canada. It was never stated which Union Station Sato was referring to.