Tube passengers and staff watched in horror as a 14-year-old boy was electrocuted after climbing onto Tube rail tracks.
A Tube train was delayed for 85 minutes while tracks were searched
The incident happened at about 2215 BST at Ealing Broadway station in west London on Thursday night.
Two youths were seen getting onto the District Line track and one is thought to have slipped and come into contact with the traction current rail.
A London Underground spokesman said the boy's death was "a tragic accident".
The boy's companion left the station via the track.
A westbound District Line train carrying 52 passengers was delayed for 85 minutes while tracks were searched for the missing teenager.
Witnesses who saw the boy's death were being interviewed by police.
The LU spokesman said: "This is a tragic accident and our first thoughts are with the youth's family. We are deeply saddened that someone so young has lost their life in this way.
The spokesman said a police investigation was under way to determine the circumstances of the accident.
The youth was thought to be part of a group of six teenagers seen at the station. CCTV footage at the station is being examined.