Speak to Doctor Carrington at Railroad HQ after completing the Boston After Dark quest. The goal of this quest is to check the status of the Augusta Safehouse. He will tell you to pick up a dead drop in a mailbox, which will be a random spot. Our quest led us to Quincy Ruins at the southeast corner of the map.
The dead drop will lead you to Kendall Hospital in Cambridge. It's now a rundown Raider stronghold, so expect a fight.
While it is a challenge navigating around a hollowed-out hospital with makeshift catwalks and missing rooms, the waypoint indicator will be sufficient enough for you to find the Augusta Safehouse document. You'll find it on a desk. Return to Carrington with your findings to complete the quest.
Note: While this is technically a Radiant quest due to the random location of the dead drop, this quest is not repeatable.