The train tracks involved in the Lewes incident which saw 20 people injured, had undergone a safety inspection just hours before the derailment.
Track engineers conducted a safety inspection on the stretch of line used by the Southern Rail train just a day before the incident.
The service travelled from London Victoria to Eastbourne before it crashed near Lewes station shortly before 4pm, leaving more than 20 passengers injured.
Southern had posted on X that engineers were examining the railway between Lewes and Haywards Heath, before confirming the train had come off the rails in that very area.
Whether the inspected section of track was the precise location of the derailment has not been confirmed.
The cause of the incident on the Lewes to Haywards Heath line remains under investigation.
The eight-carriage British Rail Class 377 Electrostar was carrying roughly 150 passengers when it left the tracks, with three of its carriages understood to have overturned.
Network Rail confirmed the service had diverted off the rails just outside Lewes, while East Sussex Fire and Rescue said crews were called at 3.55pm to the scene.
Reports indicated some passengers were trapped in the wreckage.
Three air ambulances and approximately 20 other vehicles were dispatched to the site.
Authorities urged members of the public to stay away from the area as firefighters and medical teams worked to free and treat those on board.
Yesterday was the hottest day of the year so far, with temperatures in Haywards Heath reaching 32C, conditions known to cause railway lines to buckle and warp.
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Aerial photographs of the crash site appeared to show visible distortion in the track.
Before the derailment, eight national rail operators had already issued warnings about potential disruptions linked to warm weather.
Government sources said the incident resulted in no fatalities or serious injuries, though the South East Coast Ambulance Service confirmed two patients had suffered serious injuries and nine others had sustained less severe harm.
The precise cause of the derailment has yet to be determined.
Transport Secretary Heidi Alexander described herself as "deeply concerned" by images emerging on social media, adding: "Grateful to emergency services who are on the scene supporting those impacted."
Lewes MP James MacCleary encouraged anyone worried about a passenger to contact the emergency services directly.
Helena Dollimore, who represents the neighbouring Hastings and Rye constituency, said she had spoken with Ms Alexander and asked any constituents requiring "urgent support" to get in touch with her office.
Passengers who did not need hospital treatment were taken to a nearby community centre, where partner agencies provided assistance.
Those with more significant injuries were transported to local hospitals for ongoing care.
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