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 the Enterprise, 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")