This is an unmarked location that is part of the Railroad quest "Tradecraft" and what served as the former Railroad base of operations before it fell to a major Institute attack. The location has two entrances; one is through the secret elevator in the basement of a Slocum's Joe donut shop, west of the Super Duper Mart in Lexington. The other entry point where the quest will take you is through the escape tunnel, a drainage pipe further to the west.
The main operations room, the generator room has an active Fusion Core. The room to the NW has Beds and a cooking station. Go upstairs and you’ll find an issue of the U.S. Covert Operations Manual on the table with the terminal.
In the room called Department X / R&D, you’ll find the red steamer trunk, armor workbench, weapon workbench, and hazmat suits. The vault contains the mission item, stealth boy, and mini-nuke.