The little girl, aged six or seven months, was left in a stroller at the Columbus Circle station, one of Manhattan's busiest, yesterday.
The 20-year-old woman pushed the stroller onto the platform when the doors of the train she was on slid open -- but did not get off herself.
Police launched an appeal for witnesses and broadcast close-circuit footage showing both the woman and the child.
"We had someone notify us about her whereabouts and who she was," a police spokesman told AFP.
The baby is in good health and has been handed over to child protection services.