An 18-year-old straphanger was stabbed to death on a Brooklyn subway train when his pal pulled “a stupid kid prank” and yanked the emergency brake — ticking off a knife-wielding attacker, police said Tuesday.

Isaiah Collazo of Staten Island was aboard a northbound D train near the Fourth Avenue and Pacific Street Station around 11:30 p.m. Thursday when his friend jokingly pulled the brake on the rail car, according to police.

That’s when accused killer Mark Smith, 25 — who surrendered to cops on Tuesday — got into a beef with Collazo, then allegedly pulled a knife and plunged it into the teen’s abdomen.

“So what we have is Collazo and his friends, they’re on the platform,” NYPD Chief of Detectives James Essig said. “They finally get on a train as it pulls in about a quarter after 11.

“His friend pulls the thing. This guy confronts him, ‘What the hell did you do that for?’ ” Essig said of the brake “prank,” which typically causes a delay for riders. “There’s sort of a scrum, pulls out a knife, hits him, stabs him.”

Smith then fled the station, police said.

Collazo was rushed to New York-Presbyterian Brooklyn Methodist Hospital, where he was later pronounced dead.

Essig said, Smith of Brooklyn turned himself in and requested a lawyer, adding that while the incident was not captured by surveillance cameras, “we have the after-the-stabbing video.”

Smith has several busts on his rap sheet, including for robbery in 2020, assault in 2019 and another assault in 2016, the chief said.

Essig said witnesses, including Collazo’s friends, identified Smith as the attacker.

“It’s a stupid kid prank,” Essig said of the motive.

He said one of Collazzo’s friends was also stabbed in the attack but was not seriously wounded.

The incident comes as major crimes are down in the city transit system this year through Sunday compared to the same time span in 2022, from 578 criminal complaints so far compared to 621 last year, a drop of nearly 7%, according to NYPD data.

But the last month shows an uptick of more than 25% in complaints involving six major crimes, including a nearly 60% increase in grand larcenies, which jumped 57 over the same month in 2022, compared to 91 so far this year, the statistics show.