It is one of several locations in the Weathervane quests given by Tinker Tom and is accessible through the garden terrace on top of Haymarket Mall. Alternately, one can reach it by jumping down to the freeway above it from the protruding beam on the second location of the USS Constitution while wearing power armor. This method avoids the fights in Haymarket Mall.
Once inside Haymarket Mall, make your way to the stairs to the second floor, and enter an elevator down the stairs on the second floor that will go up. Exit the elevator, and turn left. A machine gun turret is just ahead, so take a right and 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 the Haymarket Mall roof.
Once on the roof, not far to the right is the garden terrace, and within, a deathclaw. Look to the left to find the broken monorail. Use the fire escape or jump on it and make your way inside. Halfway across the monorail, you'll see the vantage spot for the Weathervane quest. A hostile protectron is on the front car of the monorail.
The broken monorail appears only in Fallout 4.