Mercer Safehouse

Mercer Safehouse is a Railroad side quest in Fallout 4.

Quick walkthrough
After the completion of the mission Boston After Dark given from Doctor Carrington if the player talks to P.A.M. it will say that, for the stability of the Railroad, it 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, the Sole Suvivor 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 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 to construct a new settlement to expand the reach of the Railroad. PAM states the settlement must be constructed and it will be a random settlement location. If having cleared all of the settlement locations it will immediately be a successful mission. Speak again to PAM. If haven't 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. Remember that barricades do not count as defenses until manned, and because there are no active settlers at this point, the player character 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 the reward.