Brake fault caused Caledonian Sleeper to overshoot Edinburgh Waverley platform
Emergency brakes were used to bring the overnight service from London Euston to a halt approximately 650 metres (711 yards) beyond its intended stopping point following the incident on the morning of 1 August.
Images showed how the train and most of its carriages had disappeared into the Calton tunnel to the east of the station, missing its platform by a great distance.
Investigators say the problem arose when the Class 92 electric locomotive hauling the train was attached at Carstairs en route to Edinburgh.
None of the 120 passengers on board were injured and there was no damage incurred to the train as a consequence of the incident, however, in a statement Rail Accident Investigation Branch (RAIB) officials have admitted the outcome could have been much worse, had there been a collision with another train.
The RAIB's statement adds: "This meant that the only effective brakes on the train as it approached Edinburgh were those on the locomotive, which were insufficient to maintain control of the train.
"The train was brought to a stand by the operation of an emergency device in one of the coaches by the Train Manager, which caused the train brakes to apply."
The investigation will examine the sequence of events in the lead up to the incident, including how the isolating valve came to be closed at Carstairs, how the train was driven and any relevant underlying management or organisational factors.
Findings will be published along with recommendations to improve future safety at the conclusion of the investigation.
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