Mercer Safehouse

Mercer Safehouse is a Railroad radiant quest in Fallout 4.

Quick walkthrough
After the completion of the mission Boston After Dark given from Doctor Carrington if the player talks to Predictive Analytic Machine (PAM) it will say that for the stability of the Railroad is vital to expand themselves with another settlement. It then proceeds to give the Sole Survivor the task of liberating a random settlement. Once the settlement has been cleared, you will be required to build sufficient defenses for the settlement. 10 defense is required to pass this step. Finally, when that is done, return to PAM for your reward.

Detailed walkthrough
The following guide follows this quest at Sunshine Tidings co-op. To start this quest, talk to PAM inside the Railroad Headquarters anytime after completing the mission Boston After Dark. She will instruct you to construct a new settlement to expand the reach of the Railroad. PAM states the settlement must be constructed at Sunshine Tidings co-op, a ghoul infested settlement located to the west of Walden Pond. Travel to this location and proceed to eliminate the local ghoul population, who are spread throughout the shacks at the location. Once the ghouls have been taken care of, proceed to the workshop, located in the central building. Construct defenses around the settlement until the settlement's defense rating is at least 10. Remember that barricades do not count as defenses until manned, and because there are no active settlers at this point, you 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 PAM, to claim your reward.