A 17-year-old girl died when she jumped onto the tracks and touched the third rail at an elevated Bronx subway station Tuesday morning, according to cops.

Laura Janneh leaped onto the southbound track at the Simpson Street 2 / 5 train station just after 8 a.m. and made contact with the electrified rail, officials said.

First responders shut down the power to try to rescue Janneh, but by the time they reached her, it was too late.

It’s unclear why the backpack-wearing teen was on the tracks, though a witness told investigators that she may have been trying to retrieve something she dropped, according to police sources.

Service was interrupted along the line for about an hour as cops investigated.

Heartbroken relatives gathered Tuesday evening at Janneh’s Longfellow Avenue apartment building — just a few blocks from the Foxhurst train station.

“All we know is she went to school this morning — happy — and then this,” said one male relative. “We are waiting for the doctor’s report.”

The family declined to answer further questions.

Relatives set up a GoFundMe page to help pay for the teen’s funeral.

Additional reporting by Sarah Trefethen