Police say the incident happened just before 9 p.m. aboard a southbound 5 train at the Lexington and East 59th Street station on the Upper East Side.
They say a 39-year-old woman was slashed on the forehead after an argument with another woman.
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Authorities say the suspect, believed to be in her 50s, took off at the 59th Street station.
She was last seen wearing an orange backpack and a grey sweatshirt.
The victim was taken to Weill-Cornell, where she's expected to survive.
This is breaking news. This story will be updated as more information becomes available.
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