Brutal video obtained by The Post reveals how a gruesome, fatal subway surfing incident on the Williamsburg Bridge in Lower Manhattan unfolded.

The clip shows the 14-year-old boy and his 18-year-old pal on the top of the J train as it zoomed along in Lower Manhattan on its way to Brooklyn Friday – before one of them is struck in the head by a beam and falls all the way to the ground below.

The other boy appeared to crouch down to avoid the low beam, a police source said. He can then be seen falling onto the rails on his back.

The pair were found on the tracks, unconscious.

The 14-year-old boy was killed and the 18-year-old was gravely injured, cops said.

The older surfer, who turned 18 on Friday, was rushed to Bellevue Hospital in critical condition.

A memorial was set up Saturday night in the lobby of the deceased boy’s building, with neighbors and friends writing messages on a piece of cardboard for him. 

“He was my best friend,” one child, who attended MS 345 with the dead boy, told The Post.  

The pair had lunch together in the cafeteria Friday.

“The last time I saw him was the day he died,” the teen said. “At lunch, we was laughing together. I was just saying to him, ‘Always be careful. Please, be careful.’ 
And he was like, ‘All right, bro, I got you.’ And then to see this five hours later, it’s crazy. Really crazy.”

The victim had just graduated middle school, a neighbor told the Post. 

“The family is hurting,” the neighbor said. “The building is hurting. He was a smart kid, a good kid that made a bad decision.”

“I just lost my son,” the boy’s father said when he opened the door. “Please let us grieve.”

Afterwards, his voice could be heard through the door: “I want the world to know, he was a good kid. He was a good kid.”

In October, a 12-year-old Brooklyn girl was killed in a similar subway surfing incident on the same span.

Zemfira Mukhtarov had frequently posted her death-defying adventures on TikTok before she was killed.