A rail track defect existed in the area where a train derailed in East Sussex last week, injuring 30 people, investigators said.
An interim report by the Rail Accident Investigation Branch (RAIB) said CCTV images from preceding services on the line show the fault, which it described as a “track geometry irregularity”.
It published an image appearing to show the track had buckled, which can happen during high temperatures.
The crash happened on the UK’s hottest day of the year so far.
The RAIB noted that there has been “a sustained period of hot and dry weather”, and the nearest weather station to the scene of the accident recorded the ambient air temperature at the time as about 27C.
Following the accident, investigators have received “several reports of rough riding” from passengers on other trains at the same location, the RAIB said.
Preliminary analysis of the derailed train’s onboard data recorder show it was travelling at about 65mph as it approached the point of derailment in Lewes at 3.54pm on August 13.
About 150 people were onboard the eight-carriage Southern train from London Victoria to Eastbourne.
Three carriages ended up on their side, trapping passengers.
A total of 30 people were injured, included 20 who required hospital treatment, the RAIB said.
British Transport Police previously said two people sustained serious injuries which were not deemed to be life-threatening.
The RAIB said its investigation will consider the weather conditions on August 13, which were hot and sunny, as well as the status of the track, embankment, condition of the train and the effectiveness of any measures taken to address potential issues caused by hot weather.
Lucy McAuliffe, Network Rail’s Sussex route director, and John Whitehurst, chief executive at GTR, which operates Southern, issued a joint statement which insisted that Britain’s railway is “one of the safest in the world”.
It is “essential” that “we understand exactly what happened” at Lewes and “identify any lessons that can be learned”, they said.
They added: “We will carefully consider any recommendations arising from the investigation and work across the industry to implement them.”
The railway between Lewes and Haywards Heath will remain closed until at least the end of service on August 26 as the train is recovered and the track repaired.
Another train came off the tracks a day after the Lewes incident.
The rear carriages of a Greater Anglia train derailed in an upright position south of Wickford railway station in Essex.
No injuries were reported.
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