An investigation has been launched after passengers on a packed rush-hour train forced open doors after it stopped for hours, partly in a tunnel, and jumped on to the track, only for the train to start moving again.
The incident happened between London St Pancras and Kentish Town stations when a First Capital Connect (FCC) service, with hundreds of commuters on board, stopped because of power problems.
The Brighton to Bedford train, which had a capacity of 476, was fully loaded, with many passengers standing, when it stopped at 6pm on May 26.
The Rail Accident Investigation Branch (RAIB) said that, by the time another train was sent to help the failed service more than two hours later, passengers had used the emergency release handles to open the doors to improve ventilation because the air conditioning was not working.
"The train began to move at 2103 hrs but this movement was immediately stopped because passengers were getting on to the railway from the carriages within the tunnel," said the RAIB.
"The investigation will identify the sequence of events that led to the movement of the failed train with open doors."
A FCC spokesman said: "We are fully supporting the RAIB investigation and have commissioned an independent investigation into this incident as well. Once our independent investigation is complete we will make the findings available to customers."
The RAIB said the train had stopped because of an electrical problem on the train itself which had caused the overhead line's electrical protection equipment to trip, cutting off power to several trains in the area.
FCC maintained it was overhead power problems that caused the train to fail and not a train failure.