The 54-year-old Brooklyn woman was slashed while on a platform inside the 14th Street-Union Square station just after 10 p.m. Wednesday.
Sean Conaboy said he was waiting for the train when he saw the knife and saw the woman being attacked. Instead of running the other way, he instinctively pounced on the man and almost certainly saved the woman's life.
Not only did he stop the attack, but he also helped catch the knife-wielding man.
This surveillance video was obtained exclusively by Eyewitness News on Thursday and shows the suspect walking toward the woman with a large knife in his right hand.
First at 7: Video shows the incident unfold:
"I was looking at the track, she's the nearest person to the edge of that platform," Conaboy said. "I noticed her. And suddenly she got yanked backwards and screamed, and this dagger or knife very quickly appeared and made an arching sweeping turn. I knew someone was being attacked."
Authorities say 22-year-old Joshua Nazario came up from behind and stabbed the woman multiple times in the back and chest.
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Conaboy didn't hesitate to put his own safety at risk and help.
"I heard a woman scream, and so that's enough for me," Conaboy said. "I just jumped him, I just tackled him, jumped on his back. And that took us down to the platform surface. I'm trying desperately to keep him down, facedown, because I know that if he gets up, or if he can turn on me, and he has that knife now, I'm a potential victim."
Bystanders piled on and, in the struggle, the knife slipped onto the tracks. Responding officers arrived moments later and made the arrest.
The victim got away and was taken to Bellevue Hospital with non-life threatening injuries. She is expected to survive.
Nazario was arrested and charged with four counts of assault and criminal possession of a weapon.
"It could be someone else next," Conaboy said. "And if things continue on the way they are, it will be someone else next."
It was the kind of random, senseless attack that has plagued the transit system in recent weeks-leaving a trail of victims and untold numbers of New Yorkers riding the trains in fear-or not at all.
This as ridership is roaring back just as the city is reopening. According to the MTA, ridership so far this year is more than three times higher compared to last year.
Despite the attacks, Mayor Bill de Blasio continues to promise a safer system.
"We are going to turn this around," de Blasio said. "The most important thing to say, I said the other day, recovery equals public safety. Public safety equals recovery. I really believe that."
Conaboy isn't so sure.
"What's your defense? Your best defense is to not be there, but we all have lives that we have, jobs, to go to," Conaboy said. "We have homes to go to, places to go to. If New York is going to reopen and expect people to go out to restaurants and theaters, you can't have this happen."
NYC Transit Interim President Sarah Feinberg released the following statement about the attack:
"This unprovoked attack is both senseless and deeply disturbing - and another reminder of why additional uniformed police officers and mental health services are needed in the transit system to both increase security and to ensure the safety of riders and transit workers. We are deeply grateful for the generous acts of each of the Good Samaritans who jumped to this victim's aid last night, at significant risk to themselves."
MORE NEWS: Video shows NYPD transit officers rescue man who collapsed onto subway tracks
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