To trigger this quest, you will have to complete the Rockets' Red Glare Railroad quest and pursue a number of Radiant quests for various members of The Railroad. Eventually, P.A.M. will ask you to hunt down one of the very few survivors of The Brotherhood of Steel.
As this is a Radiant quest, the designated location will be random. Wherever it is, head there to confront the Brotherhood troops. Kill them and report back to P.A.M. to complete the quest.
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