Mercer Safehouse

Mercer Safehouse is a Railroad side quest in Fallout 4.

Detailed walkthrough
To start this quest, talk to P.A.M. inside the Railroad Headquarters anytime after completing the mission Boston After Dark. She will instruct the player to construct a new settlement to expand the reach of the Railroad. P.A.M. states the settlement must be constructed and it will be a random settlement location. If one has already cleared all of the settlement locations it will immediately be a successful mission. Speak again to P.A.M to receive a reward.

If one has not cleared all locations, travel to the random location, and proceed to clear the area, then construct defenses around the settlement until the settlement's defense rating is at least 10. Barricades do not count as defenses until manned and, because there are no active settlers at this point, the player will be required to construct turrets or traps to gain the necessary 10 defense. After this task is complete, head back to the Railroad headquarters and proceed to talk to P.A.M., to claim the reward.