A Brooklyn man charged with fatally stabbing a teenager during a fight on a train last month was found to have acted in self-defense — an outcome legal experts said may play out again in the subway chokehold case against Daniel Penny.

Mark Smith, 25,had been charged with manslaughter for stabbing 18-year-old straphanger Isaiah Collazo to death during a beef sparked when the teen’s pal pulled the emergency brake aboard a D train on April 6.

But a grand jury on Monday voted not to indict the accused killer — finding his conduct during the fatal encounter was justified — and the case against Smith was dismissed, the Brooklyn District Attorney’s Office confirmed.

At least one witness to the incident told the grand jury it was Collazo and his friends, and not Smith, who were the aggressors during the lethal conflict, sources said.

“The facts of this case never inculpated Mr. Smith, and this outcome is consistent with his role in the incident and the law,” Martha Lineberger, staff attorney with the Legal Aid Brooklyn trial office, which represented Smith, said in a statement Tuesday.

“The Brooklyn DA’s office conducted a thorough investigation, with Mr. Smith testifying directly to the grand jury, resulting in this just dismissal,” Lineberger added.

Barring new evidence, prosecutors are unable to bring charges in the case, which is now sealed.

Legal experts noted some similarities between this case and the one against Penny, the 24-year-old ex-Marine charged with manslaughter in the May 1 choking death of Jordan Neely on a Manhattan F train. Penny was released on $100,000 bail at his arraignment Friday.

The Manhattan District Attorney’s Office must now try to secure a grand jury indictment before the case can proceed.

Attorneys for Penny have argued he was merely trying to defend himself and fellow straphangers from a threatening homeless man when he put Neely, 30, in the chokehold.

“There is no doubt that Daniel Penny’s legal team is looking for a similar result and are actively seeking both video evidence and eyewitness testimony aimed at establishing that the decedent was acting in a threatening manner and that others feared for their life,” defense attorney Joseph DiBenedetto told The Post Tuesday.

“I am sure that they have subpoenaed video and have investigators locating and speaking to as many eyewitnesses as possible,” he said. “This evidence is crucial for establishing that Penny’s actions were justified under the circumstances.”

Shocking bystander video showed Penny holding Neely in the chokehold until he stopped moving on the train floor.

In Smith’s case, police officials have said the incident was not captured by surveillance cameras, but that investigators obtained video showing the aftermath of the stabbing — although it is not clear what, if any, footage was presented to the grand jury.

Michael Discioarro, a former Bronx prosecutor and now prominent defense lawyer, said it’s “absolutely conceivable” that the grand jury panel members “put themselves in the defendant’s shoes in Smith’s case — and the same could hold true when another grand jury weighs charges against Penny.

“This is exactly how it is going to play out with Penny,” Discioarro said. “Grand jurors will ultimately decide who they believe. That’s the jury’s job. Facts, credibility and evidence.

“Especially with a self-defense claim. Penny may testify. He may not. But the process will be the same.”

Meanwhile, defense attorney Jason Goldman said the Brooklyn grand jury clearly believed Smith’s story when he testified to the panel — and the same could hold true in Penny’s case.

“The same outcome could certainly take place with Penny’s case,” Goldman said. “Interestingly, Smith testified at the grand jury, which clearly played well and fostered the dismissal.

“It is always a gamble but Penny and his team may elect to do the same,” he said.

Smith, who fled the station after stabbing the Staten Island teen, surrendered to police several days later.

The victim’s father, Carlos Collazo, said he was puzzled by the grand jury’s decision.

“I’m confused about the whole situation,” he told CBS News. “My son definitely didn’t deserve this.

“This guy didn’t even get in trouble,” he said of Smith. “He walked out like a day or two after he killed my son and stabbed two of his friends.”

Additional reporting by Joe Marino