Smoke was spotted billowing out of a train, causing disruption to holidaymakers trying to get to Gatwick Airport.
Southern Rail said there were reports of a fire on an empty train at Clapham Junction, with railway lines between Balham and London Victoria blocked.
A spokesman said: ‘Following reports of smoke from an empty Southeastern train travelling to a depot, engineers attended and identified the cause as a brake fault.
‘The issue has been resolved, the train has moved safely and all lines have reopened.’
The incident disrupted Southern and Gatwick Express services between Balham and London Victoria.
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There have been no reports of any injuries.
At the same time, the Windrush Line is facing severe delays between Highbury and Islington and Crystal Palace ‘due to an unsafe structure near the railway’.
There is no service between Surrey Quays and New Cross.
Work is still underway to recover overturned carriages from the Lewes derailment that injured two people last week.
On Thursday, around 150 passengers were travelling on the London Victoria to Eastbourne service when three carriages came off the tracks.
With an investigation underway into the exact cause of the incident, which those onboard described as ‘terrifying’, Network Rail has now said the Lewes to Haywards Heath line will remain closed until next week.
The first carriages have been seen being recovered and towed to a depot as work continues to clear the line.
This is a breaking news story… More to follow…