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.
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.
Ex-Brit turned Manhattan resident since 2008.