Commuters were put at risk when they forced open the doors of a packed train and walked on the track after being stuck in a tunnel for more than three hours in searing heat.
An investigation has been launched into the incident which took place on the Brighton to Bedford service between St Pancras International and Kentish Town on May 26.
After forcing open the doors, the train began to move and when the passengers re-boarded, three sets of doors remained open for the next mile into the next station.
More than 500 passengers were onboard the train which stopped during the rush-hour at 6pm.
Temperatures soon rose because the ventilation system failed due to problems with the overhead cables.
After being delayed for several hours, some passengers used the emergency release handles to open the doors to let air in. Some of them started walking onto the tracks.
The Rail Accident Investigation Branch said: “The train began to move at 9.03pm but this movement was immediately stopped because passengers were getting out on to the railway from the carriages within the tunnel.”
A spokesman for operator First Capital Connect said: “We are fully supporting the RAIB investigation and have commissioned an independent investigation into this incident as well. This will be made available to passengers.”