It would be a random victim: the shooter would have first walked up and down the subway car before pulling out his firearm.
In April, New York was also rocked by a shooting in the subway, when a man opened fire during the morning rush hour. Just before that, he had put on a gas mask and set off smoke bombs. Ten people were injured by the bullets and another thirteen others were injured in the ensuing panic. There were no deaths. The perpetrator, Frank James, was arrested a day later.