Network Rail and Southern owner GTR have apologised after an interim investigation into last week's train derailment near Lewes, East Sussex, found that a "track geometry irregularity" existed on the approach to the crash site before the incident occurred. The Rail Accident Investigation Branch (RAIB) published CCTV imagery appearing to show the track had buckled, which can happen in extreme heat, on a day when temperatures reached around 27C near the site. The derailment injured 30 people and has prompted safety concerns about Britain's rail infrastructure coping with sustained hot weather.

The train, carrying about 150 passengers, was travelling at roughly 65mph, within the 70mph limit, when three carriages came off the track and ended up on their side. Investigators noted the embankment is made of clay, which is sensitive to moisture changes, though found no evidence of a significant slip, and said they had received prior reports of "rough riding" from other trains on the line. The incident occurred on 13 August during the UK's fifth heatwave of the summer, a day before a second, less serious derailment involving a Greater Anglia train at Wickford, Essex. The Lewes-to-Haywards Heath line is expected to stay closed until at least 26 August to allow for recovery of the train and preservation of evidence.