4. Three Ways to See the Old City Hall Subway Station

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The official way to visit the old City Hall subway station is to join the New York Transit Museum and get on the tours they run there twice a year. Tickets usually sell out in about an hour so you need to be on top of your game.
Alternatively, you can stay on the 6 train and ride through the turnaround as the train shifts from downtown to uptown. If the lights are on in the station.