http://youtu.be/8blDeM8RJU8 Explained by the guy behind the camera (Jinais).
A usually boring Monday morning commute to work at Brooklyn was stirred up by this little guy. He joined us at the Fulton street station on the A train going downtown. Someone getting off the train was screaming „RAT on the train!“. But by the time everyone realized what was happening, the doors closed and the train entered the tunnel. We were stuck with him till the other end.