A New York City Transit employee was killed overnight by a southbound D train while a crew of contractors was preparing scheduled work on a Bronx subway line, the MTA said Friday.

Moet McCullum, a 41-year-old mother of three, was preparing to set up flags on the tracks between 167th St. and 161st. St. along the D line in Highbridge when she was hit by an oncoming train shortly after 11:30 p.m. Thursday, multiple transit sources told the Daily News.

McCullum was in the process of setting up the flagging lights that warn oncoming trains to the presence of a work crew. The D train operator reported not seeing any such lights before striking McCullum, according to sources.

The track worker was taken to Lincoln Hospital, where she died.

“This just broke my heart, to wake up in the early morning hours to find out that we had lost one of our dearest members,” John Chiarello, head of Transport Workers Union Local 100, said Friday.

“She had 11 months on the job,” he said. “Our prayers go out to her family — her three children, young children; Elizabeth, her mother; and Monet, her twin sister, who is also a conductor.”

“It’s inherently dangerous work,” said Chiarello, who rose to union leadership after being a signal maintainer for NYC Transit. “When you’re working under live service, keeping New York moving, you’re going to have issues.”

“Not only was a life lost, but the train crew that were involved will be traumatized for the rest of their lives,” Chiarello added.

NYC Transit President Demetrius Crichlow was among the MTA brass who joined union officials at a press conference Friday.

“This is a difficult day,” Crichlow said. “Nothing is more important than safety, the safety of our employees and the safety of our customers. So we stand together with the union and with the investigating agencies to ensure that the root cause is found and we do what is necessary to protect our employees.”

“I met with the family this morning,” Crichlow said. “It’s just a terrible situation.”

In a social media post Friday morning, the National Transportation Safety Board said it was sending a team to New York to investigate McCullum’s death.

“Flagging” — the act of warning oncoming trains that there are track workers up ahead — is regarded as among the more dangerous tasks in subway track work.

NYC Transit worker Hilarion Joseph was also in his first year on the job when he was struck by a D train and killed while flagging for a track-cleaning crew in November 2023.

Another flagger, not publicly identified, suffered severe head trauma in 2024 after he was hit by an out-of-service F train in Brooklyn.

Both incidents were cited in a scathing Federal Transit Administration assessment in 2024, which demanded that the MTA improve its safety procedures and that the state’s Public Transportation Safety Board step up its oversight of New York City Transit.

The MTA appealed that report, saying the findings mischaracterized the agency’s safety record.