Check out this exclusive footage of the 6 train turning around in the no-longer-used City Hall subway station -- one of the city's first stations! Note that the station is well preserved and not "abandoned" as much as it's no longer in use for straphangers. This is due to the large gap that is present between train doors and the platform (due to the circular tracks). That's why it's clean and gorgeous and litter- and graffiti-free!
The station allows the downtown 6 trains to turn about and come back up on the uptown track. Make sure to put earplugs in because it's LOUD in there!!! Listen to that train screech! I was lucky to be given a special tour via the MTA, so I'm not encouraging sneaking in though I imagine if you just "forgot" to get off the downtown 6 at City Hall, no one could bust you for riding the train as it circles through and heads back uptown.