The New York City subway system has a lot of interesting art. But as far as I'm aware, there's only one artwork inside one of the tunnels: Bill Brand's Masstransiscope, which was recently restored to its 1980s glory.
If you ride the B or Q trains from the DeKalb Avenue stop in Brooklyn towards Manhattan, look for this artwork out the right side of the train. Through a series of openings in the wall, you'll see images that, when the train is moving, appear to be moving also. You can get a sense of it in this video.
Brand, a professor of film and photography, says of the work that he "started thinking about it in grad school in Chicago while riding the L, looking out the window of the train and thinking about early cinema." Masstransiscope works on the same principle as a zoetrope, which is a spinning tube with slits cut out that is lined with pictures. You watch the images through the slits, which keep the images from just blurring together—so basically, a zoetrope is a simple movie.
The B and the Q are often slowed or stopped beside the work because of a signal just ahead, but Brand says, "I built that into the idea for the piece. It breaks the illusion."
Now I'm wondering: What other cities have great subway art, in either the stations or the tunnels?