Once you enter the Railroad HQ, Drummer Boy may approach you and tell you that Desdemona needs to talk to you. Talk to her and she'll ask you to check the Ticonderoga Safehouse.
Upon entering the building, you'll find several synths and a named courser, X9-27. They won't be immediately hostile on sight. X9-27 will talk to you but there won't be an option for them to stand down or send them away. To proceed with the quest, attack the courser and destroy all synths in the floor. Make your way down the building where you'll have to deal with another courser and more synth reinforcements. Report back to Desdemona to complete the quest and receive the rewards.