This is a Railroad side quest obtained from Tinker Tom.
This Weathervane is located on a Broken Monorail, and accessible by entering Haymarket Mall. Haymarket Mall is Southwest of Pickman Gallery, and Southeast of Cabot House.
Once inside Haymarket Mall, make your way to the stairs to the second floor, and enter an elevator that will take you up. Exit the elevator, and go through the door on your left. You'll see a machine gun turret just ahead.
Go through the door on your right, then up a ramp to the floor above. There is another machine gun turret there, and a door to the left. The door leads to Haymarket Mall Roof.
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Once on the roof, head to your right and you'll discover Garden Terrace, and encounter an Alpha Deathclaw. Look to your left to find the Broken Monorail. Jump on the monorail and make your way along it, be careful not to fall. Half way across the monorail you'll see the vantage spot, and inside the monorail will be a Protectron Guardian.
Return to Tinker Tom to complete the quest.
