The entrance is through a Slocum's Joe which is partially blocked by a crashed bus. The interior is reached via an elevator and is completely ruined aside from a few containers with random loot. All doors are inaccessible. There is another elevator inside leading to the roof of the building, which leads to the USS Constitution after the quest Last Voyage of the U.S.S. Constitution, which has crashed into another establishment, the Baxter building.
The Baxter building only appears in Fallout 4.