It happened at the 175th Street station in Washington Heights around 11 a.m. Sunday, on the southbound A train platform.
Authorities say the 46-year-old victim was waiting for the train when he was struck in the head from behind with the hammer.
Before the attack, the suspect reportedly yelled, "I'm going to (expletive) someone up."
The victim, Patrick Osborne, said he could tell the suspect was angry about something.
"Maybe he make a mistake and took me for somebody else, that's what I was thinking at the time," Osborne exclusively told Eyewitness News on Tuesday. "He said something like he's gonna bash somebody or something like that, he's gonna bash somebody in the head or something. I thought he was just having like an earphone or phone or talking to one another."
He said he saw the suspect holding a hammer but otherwise there was nothing about him that seemed dangerous.
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Osborne said he was facing the southbound A train and had his back to the suspect -- he never saw what was coming.
Osborne managed to get to a token booth clerk attendant for help. He was treated for head pain at the scene but refused further medical attention.
He suffers from headaches and says he injured his knee when he fell to the ground.
Osborne said he is thinking twice about riding the subway now. And when he does, he has flashbacks.
After striking Osborne, police say the suspect ran away from the scene.
Anyone with information in regard to this incident 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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