She's gorgeous, she's talented: she's the singing siren of Third Rail and I bet you had no idea you can even talk to her, much less share a night of post-apocalyptic passion. Whether your character is male or female, you can romance and woo Magnolia until she asks if you'd like to go on a date. Head over to The Cabot House in northeast metropolitan Boston to get things started.
At the entrance to the Cabot House is an intercom, which you can use to speak with the inhabitants to gain entrance. They will send you on a series of three missions, the second of which will require you question Magnolia about the disappearance of a Cabot family member. The robot bartender will call her off the stage to speak with you. Before you approach, equip any high Charisma items you have in your inventory, like Agatha's Dress or Reginald's Suit or use Charisma boosting meds like Daytripper (which is particularly helpful because it also enhances Luck, which Magnolia seems to respond to).
Start off by complimenting her music.
Tell her how much you enjoyed her song and she will light up and become excited. This will open up the flirtacious dialogue exchanges. After three successful speech checks, you will be able to go on a date with her. You two head over to the Hotel Rexford.
Afterwards, she'll tell you that she can't get too attached but will always think of you as she sings on stage. You will need to revisit the Third Rail to pick the line of questioning back up so you can continue to search for Emogene Cabot and track her down.
And wouldn't you know it, after we had sex and I was doin' the walk of shame, I ran into a very, very old friend. Typical. If you haven't visited Hotel Rexford yet, this should give you extra incentive. Recruiting the Vault Tec Rep for your settlements and placing him at a weapons shop will allow you to purchase a few unique pieces of armor. Go visit him and say hello on your way out.