The RAIB is investigating an incident which took place between London St Pancras International (Low Level) and Kentish Town stations, on 26 May 2011.
At approximately 18:00 hrs the  FCC 16:30 hrs Brighton to Bedford service, formed of two Class 377 units (a total of eight carriages), stopped close to the northern portal of Kings Cross Tunnel with the rear two carriages within the tunnel.
The train had stopped because of an electrical problem on the train itself that had caused the overhead line’s electrical protection equipment to trip, cutting off power to several trains in the area.
At this time the train, which had a seating capacity of 476, was fully loaded, with many standing passengers.
Passengers described the ordeal as ‘hideous’ and one lady, who was pregnant, collapsed on board after saying she felt ‘dehydrated and claustrophobic’.
Another train of the same type was sent to assist the failed train, and was eventually coupled to it at 20:20 hrs.
By this time some passengers had used the emergency release handles to open the train doors in an attempt to improve ventilation (the air conditioning and lighting systems were no longer functioning).
The train began to move at 21:03 hrs but this movement was immediately stopped because passengers were getting out onto the railway from the carriages within the tunnel.
The passengers were escorted back onto the train, which was then authorised to move forward again at 21:12 hrs.
Three sets of doors towards the rear of the train were still fully open while it travelled approximately one mile to reach Kentish Town station, where all the passengers left the train.
The RAIB investigation will identify the sequence of events that led to the movement of the failed train with open doors. Factors for consideration will include the design and operation of the trains, the management of the incident and the information available to the passengers and staff who were involved.
The RAIB’s investigation is independent of any investigations by the British Transport Police or the safety authority (the Office of Rail Regulation).
The RAIB will publish a report, including any recommendations to improve safety, at the conclusion of its investigation.
There needs to be better, and effectively rehearsed, contingency plans to deal with such events. Where there are valid reasons for not being able to move a train, steps must be taken to allow those passengers capable of doing so to detrain safely and walk to the nearest station.
Failing that, some system of making supplies of bottled water and snacks available to passengers ought to be in place.