The old City Hall subway Station, New York.

The first New York City subway was built and operated by the Interborough Rapid Transit Company (IRT) and opened on October 27, 1904, to the joy of its inhabitants.
City Hall was the ceremonial terminal, the place where the mayor could show off the subway built with the people’s money to benefit the greatest city in the country.

The City Hall station on the IRT local track was embellished with fine architectural details, including a sharply curved platform, a Guastavino tile arched ceiling, brass chandeliers (blackened in World War II), and plaques praising the work and those involved. The plaques were hung on the track-side wall commemorating the Interborough Rapid Transit company and honoring the Rapid Transit Subway Construction Co., listing the directors, engineers, and financiers, including August Belmont, Cornelius Vanderbilt, and John D. McDonald.
However, the vaulted ceilings and other details were lost on busy commuters, and the stop was one of the least-used in the system.
The official start of construction took place on 24 March 1900 at the front steps of City Hall, on the site of the station.

Unfortunately, It was located on the turning loop for local trains from uptown, and both those and the express trains could be easily taken at the very nearby Brooklyn Bridge station. The curve was noisy, and the gaps it left at the platform were unsafe. The station was closed at night, while the local trains ran to South Ferry loop instead. When it was open, it was an entrance only. In addition, It was the only station that did not have turnstiles installed by 1923.
Because of the curved platform, cars with center doors could not be used at this station unless they had specially modified door controls which allowed just the end doors to be opened.

City Hall was closed to passenger service on December 31, 1945. The street entrances were sealed and the skylights covered over when platforms along the line were being lengthened to accommodate longer trains, and the number of passengers using this station dwindled to very few. In its last year, a peak year for subway travel, only 600 people a day used it, a very small number in New York.

Up until the late 1990’s the passengers on the Lexington Avenue Local (the current 6 train) had to disembark from the train at the Brooklyn Bridge stop. The loop track is of course still very much in use. Every run of the 6 train terminating at Brooklyn Bridge uses the loop to run around to the uptown local track.
Today the skylights have been reopened, and the station lights turned back on. While passengers can not get out of the train and experience the City Hall Station as they once might have, they can stay on the train as it loops around on those tracks and heads back north.
The New York City Transit Museum hosts periodic tours of the abandoned station, even if you must be a member of the museum to attend. This is the website for details.
A personal suggestion: If you will be staying on the downtown 6 train after it leaves the Brooklyn Bridge station, it is best to be in the 7th, 8th or 9th car, beacuse the train operates very slowly around the curve in front of the City Hall station…;)

Images from Web.