The 38-year-old woman was working on a southbound train when she was attacked at the Prospect Park subway station on Saturday just before 6:30 a.m.
The unprovoked assault occurred while the train was boarding passengers in the station.
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The victim suffered pain and bruising to the face.
EMS responded to the station but the victim refused medical attention at the scene.
The suspect is described as a male in his 20s to 30s.
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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