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 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 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.
Once on the roof, not far to your right, you'll discover the garden terrace, and encounter a deathclaw. Look to your left to find the broken monorail. Use the fire escape or jump on it and make your way inside. Half way across the monorail you'll see the vantage spot for the Weathervane missions.
A hostile protectron is on the front car of the monorail.
The broken monorail appears only in Fallout 4.