Rail accident investigators said on Wednesday that a track irregularity was visible in CCTV images taken immediately before the derailment of a passenger train in England last week that seriously injured two people.
"A track geometry irregularity existed on the immediate approach to the point of derailment, prior to the accident occurring," a Rail Accident Investigation Branch statement said.
"There was also evidence of previous track maintenance activity in this area."
The accident, near Lewes in southern England, happened during a period of sustained high temperatures in Britain.
The ongoing investigation will consider factors including the effectiveness of any hot weather mitigation and the condition of the train, track and embankment.
Wednesday's statement said the railway embankment at the site was constructed of material that can be sensitive to changes in water content.
However, it added: "Although, at this stage, there is no evidence to indicate that a significant embankment slip occurred before the accident, there is evidence that some movement had previously taken place."
A full report will be published once the investigation is complete.
