The incident was the second time this month that a person was pushed on to the subway tracks by a fellow commuter.
New York police commissioner Raymond Kelly said the police are still searching for the female suspect, who raced down two flights of stairs after the attack and disappeared into a crowd on the street.
Sen had opened a small copying and printing business this year in the city's Upper West Side. He was hit by the first car of the train and his body was pinned under the second car before the 11-car train came to a stop.
The condition of Sen's body had made it difficult for the police to identify him and investigators used his phone and a medicine bottle he was carrying for his identification.