A police source told Fox 5 New York that two MTA workers on the R train were having some sort of dispute with a passenger Tuesday night.
One of the MTA workers had a gun. When the train pulled in to the Union Street station in Park Slope, the dispute continued onto the on the mezzanine level.
The armed MTA worker shot the passenger, the source said, but it isn't clear why.
The passenger is in critical condition at Methodist Hospital.
Video from Citizen.com showed a very active scene outside the station.