The incident happened around 4:30 p.m. Feb. 14 on a northbound 1 train near the 34th Street/Penn Station stop.
Authorities say the 60-year-old victim was exiting the train when the suspect said, "God bless you, stay safe."
Police say the woman stabbed the man multiple times in the back with an unknown sharp object.
The victim said he immediately felt pain around his spinal cord and kidney area after that. He fell to the ground and the female suspect left the scene.
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The victim sought medical attention the following day.
Ridership on the New York City subway is still way down from a year ago, but the number of passengers is slowly increasing as crime continues to rise.
The recent spree of violence includes stabbings this month that left two people dead and two wounded.
In response to the attacks, Mayor Bill de Blasio and NYPD Commissioner Dermot Shea have deployed 600 NYPD officers to patrol the subways. Still, Interim Transit President Sarah Feinberg says she wants more.
In the Valentine's Day attack, police say the suspect is described as a woman who is 5 feet 6 inches tall and was last seen wearing a blue mask, a black knit hat, a gray coat, black pants and carrying a black shoulder bag.
Anyone with information is asked to call the NYPD's Crime Stoppers Hotline at 1-800-577-TIPS (8477) or for Spanish, 1-888-57-PISTA (74782).
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