Rail investigators say they have identified track irregularities close to the scene of last week’s train derailment near Lewes.
Thirty people were injured when the train left the tracks on 13 August, including two who were critically hurt.
The Rail Accident Investigation Branch has said its preliminary findings also identified evidence of previous maintenance work in the area.
However, investigators have not said whether the recent hot weather played any part in the derailment, and work to establish the cause is continuing.
The findings come as Rail Minister Lord Peter Hendy personally thanked emergency services, railway staff and local volunteers for their response to the incident.
Lord Hendy visited the Tenants Resource Centre in Lewes on Tuesday, 18 August, where he met representatives from organisations involved in the emergency response.
They included Network Rail, GTR, South East Coast Ambulance Service, East Sussex Fire and Rescue Service, West Sussex Fire and Rescue Service, British Transport Police, East Sussex County Council, Lewes District Council and the Landport Road Community Centre.
Emergency services, railway staff and volunteers provided medical treatment, shelter and refreshments to passengers following the derailment.
Lord Hendy said: “I want to pay tribute to everyone who responded so quickly and professionally to the incident outside Lewes last week.
“The hard work of our emergency services and railway staff, combined with the extraordinary community spirit shown by the people of Lewes, meant that passengers received the support and care they needed at a difficult time.
“My thoughts remain with those who were on board and everyone affected by this incident.
“We will continue to work closely with the rail industry and investigators to help restore services for passengers, understand what happened and establish whether any lessons can be learned.”
The Rail Accident Investigation Branch deployed a team to the site following the derailment and is carrying out a full independent safety investigation.
Network Rail and Southern are continuing work to recover the derailed train and repair the railway.
The line between Lewes and Haywards Heath remains closed, with replacement buses operating while the work continues.
Passengers are being advised to check their journeys before travelling.
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