A train derailment that injured 30 people happened after a track appeared to have buckled on what was the hottest day of the year so far.
The Rail Accident Investigation Branch (RAIB) said its investigation into the crash in Lewes, East Sussex, on August 13 will consider the weather conditions.
It released a CCTV image showing the bent track where the incident happened.
Investigators described the fault – recorded by cameras on preceding services – as a “track geometry irregularity”.
August 13 was the UK’s hottest day of 2026 so far, with temperatures reaching 38.1C in west London.
The RAIB noted there was “a sustained period of hot and dry weather” before the accident.
There is a risk of rails buckling during high temperatures as they can be 50C when the air temperature is 30C.
After the accident, investigators received “several reports of rough riding” from passengers on other trains at the same location, the RAIB said.
Preliminary analysis of the derailed Southern train’s onboard data recorder show it was travelling at about 65mph as it approached the point of derailment at 3.54pm on August 13.
About 150 people were onboard the eight-carriage service 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 as well as the status of the track and embankment, the 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 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.
James MacCleary, Liberal Democrat MP for Lewes, said the preliminary findings “raise serious concerns”.
He said: “A pre-existing irregularity on the track is deeply concerning.
“I travel this line very regularly and reported to the authorities just how bumpy this section of track had become.
“We cannot assume that this is related, but that makes the current findings even more troubling.
“This should also be a wake-up call about the chronic underinvestment in rail infrastructure across the South East.”
A Department for Transport spokesperson said: “Safety remains the absolute priority across our rail network, and we will ensure the industry acts on any findings in the Rail Accident Investigation Branch’s full report when published.
“The UK’s railways remain among the safest in the world, with the number of delays caused by track faults falling by 45% since the late 2000s.
“We’re investing £29 billion to renew and maintain the network and a further £2.6 billion to improve climate resilience.”
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.
No injuries were reported.
