Police say the "heinous and unprovoked" attack happened near 52nd Street on a 7 train in Queens.
Video posted online by the SBA shows the two victims, 71 and 73 years old, desperately try to avoid harm.
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Patrick Chambers, 46, is under arrest and charged with multiple counts, including felony assault, menacing and criminal possession of a weapon.
Detectives say he was in possession of the knife used in the attack.
Both victims were taken to the hospital but are expected to be OK.
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