How to unlock: automatically after completing the Mercer Safehouse quest, in Old North Church (M6,2).
Another quest which you will receive from the PAM robot in the Railroad HQ. You must secure the secret stash in the Medford Memorial Hospital. Inside you will face large numbers of Super Mutants on medium level.
You don't need to fight everyone - simply go to the top floor and reach the hidden stash. It's hidden behind a wall, if you walk close to it, it will reveal a hidden warehouse, mostly with large amounts of ammunition. You don't need to take everything - once you enter inside, you will receive new objective. Go back to PAM.
Quest rewards: experience, 300 bottlecaps, Covert Sweater Vest