Rigoberto Lopez is charged with two murders and two attempted murders at stations in Manhattan and Queens.
City police posted a photo of a knife. All of the victims are said to be homeless people.
US media report the suspect, 21 and himself homeless, was arrested wearing blood-soaked clothes.
The four attacks on the A train began on Friday morning:
In the first assault, a 67-year-old man was stabbed at the 181st Street station in Manhattan, and is now being treated in hospital
