The boy was climbing a J-train headed over the Williamsburg Bridge when he slipped and fell onto the third rail, according to police.
He was pronounced dead at the scene.
A rescue train was deployed and removed about 700 passengers from the subway train, which was stopped for the investigation.
New York City Transit President Richard Davey offered sharp words on the topic of subway surfing after the incident was reported.
"Riding on top of subway cars is reckless, dumb and dangerous, frequently leading to tragedy for the person, family and friends," Davey said. "We implore parents to speak with their children about what can seem like a game but obviously is not."
The identity of the child was not released.
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MTA considering higher fare, toll increases
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