Train investigators looking into last week's derailment in Lewes said an "irregularity" existed on the approach to the site where the accident took place.
The Rail Accident Investigation Branch (RAIB) says the "track geometry irregularity" was visible in CCTV images taken immediately before the derailment in East Sussex.
An image used in evidence was taken from a train moving in the opposite direction showing the irregularity.
Around 150 people were on the train when it derailed, with 30 people were injured in the accident and 20 needing hospital treatment, some with serious injuries.
Three carriages ended up on their side, trapping passengers inside.
Significant damage was also caused to the train and to railway infrastructure, the report said.
British Transport Police previously said two people sustained serious injuries which were not deemed to be life-threatening.
Since the incident, RAIB said it has received "several reports of rough riding" at the same location from passengers on other trains.
In a joint statement, Lucy McAuliffe, Network Rail's route director for Sussex, and John Whitehurst, chief executive of Greater Thameslink Railway, said they are "very sorry that this happened on our railway".
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They added: "We will carefully consider any recommendations arising from the investigation and work across the industry to implement them, with the shared goal of making an already safe railway even safer for our customers, colleagues and the communities we serve."
Did hot weather play a role?
August 13 was the UK's hottest day of 2026 so far, with temperatures reaching 38.1C in west London.
The report added that the accident took place "during a sustained period of hot and dry weather" and that the ambient air temperature in the area was around 27C at the time of the derailment.
It also says that the railway embankment at the site was constructed of material that can be sensitive to changes in water content.
The RAIB said that, at this stage, there is no evidence that a significant embankment slip occurred before the accident but there "is evidence that some movement had previously taken place".
The ongoing investigation will consider factors including "the effectiveness of any hot weather mitigation and the condition of the train, track and embankment".
A full report will be published once the investigation is complete.
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 station in Essex, however no injuries were reported.
(c) Sky News 2026: Track 'irregularity' found at site of Lewes train derailment, investigators reveal
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