If you remain on the downtown 6 train after the last stop (Brooklyn Bridge-City Hall), you get a glimpse of the abandoned City Hall station [1] as the train turns around. This was one of the very first NYC subway stations to open in 1904!

It's a beautiful piece of history. [2]

[1] http://en.wikipedia.org/wiki/City_Hall_(IRT_Lexington_Avenue...

[2] http://www.youtube.com/watch?v=vWF3IDk9Gek