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

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.