This quest can be started after completing the Mercer Safehouse quest. With the safehouse now in Railroad hands, the place now has a caretaker. When you say hi to him, remember to skip the small talk and use the Railroad countersign. This will initiate the quest.
As a Radiant quest, your assigned location will be random. Our quest took us to Monsignor Plaza.
Simply eliminate the hostiles and report back to the caretaker to complete the quest.