A defect in a railway track has been discovered where a train derailed last week near Lewes, East Sussex, investigators say.
Thirty people were injured, two seriously, when three carriages derailed and tipped over, tumbling part-way down the embankment.
Police declared a major incident when passengers were trapped inside the train in scorching temperatures.
open image in galleryThe Rail Accident Investigation Branch (RAIB) now says rear-facing CCTV images from trains on the day showed a “track geometry irregularity” on the “immediate approach to the point of derailment” on the line just outside Lewes railway station.
Investigators also say they have received reports from passengers on other trains of “rough riding” at that spot.
About 150 people had been on the train, which was travelling from Haywards Heath to Lewes on Thursday.
One survivor later described the experience as “completely terrifying”.
“There was a banging. Everything just went topsy-turvy,” said 56-year-old Yana Ludwig, from the United States.
open image in galleryRob Bradley, who was on board with his young daughter, told how his carriage “jumped upwards” as passengers were thrown forwards.
“The train kind of skidded for quite a while before it stopped.
“Only then, when we turned around, did we see the whole back of the train was open and smoke and dust were pouring into the carriage.”
Thirty people were injured in the incident, according to the RAIB, 20 of whom needed hospital treatment, while “significant damage” was caused to the train and railway infrastructure.
The accident happened at the height of last week’s heatwave, when temperatures in the southeast reached the high 30s.
RAIB said its investigation will consider the weather, 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.
The railway line will remain closed for at least another week.
Network Rail and Southern Rail said the carriages that slid down the embankment will “need to be carefully lifted away by crane”.
They said that the location makes this “particularly difficult” as the railway is close to a river, in a site of special scientific interest, and restricted road access was via a residential area.
Once the train has been removed, engineers will be able to assess the damage to the railway and “understand the scale of the repair work needed before services can resume safely”.
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