This repeatable Railroad quest becomes available after completing The Nuclear Option. Speak to Desdemona, who will give you details about one of their remaining adversaries, The L&L Gang.
Your first target is a Raider named Lucky Tatum, although the location of his gang will be randomly selected. We ended up at the excavation site called Dunwich Borers. If your location is anything like ours, expect a lot of Raiders.
Simply follow the waypoint to Lucky. Kill him and report your success to Desdemona. You can repeat this quest five more times.