The government has launched an investigation into how a derailed freight train travelled 2.5 miles before the problem was flagged up to the driver.
The rail accident investigation branch (RAIB has confirmed it will look into the incident, which took place on the Barking to Gospel Oak line on the morning of Thursday, January 23.
The freight train, made up of 22 wagons carrying construction soil, was travelling from Barking to the Buckinghamshire village of Calvert when it derailed on a bridge near Wanstead Park station.
It was not until it had travelled 2.5 miles that the braking system alerted the driver to a potential problem.
The train then stoped between Leyton Midland Road and Walthamstow Queens Road, where it was discovered that one set of wheels on the 16th wagon had derailed.
It caused extensive damage to the track, other railway infrastructure and the wagon, with Overground services suspended between Barking and South Tottenham as a result.
The RAIB's investigation is set to look at the condition, inspection and maintenance of the track, including the bridge timbers supporting the rails.
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It is being carried out independently of any others that the railway industry or its regulator, the Office of Rail and Road, may carry out into the incident.
Network Rail has previously announced that work to repair the damage could take several weeks.
Across the two and a half mile stretch, there are 39 new pieces of rail - each 216m long - that require replacing, as well as 5,300 concrete sleepers and 900 wooden sleepers.
Work to replace 10,000 tonnes of ballast will need to take place, as well as putting replacement timber into 10 bridges.
Ellie Burrows, Network Rail's route director for Anglia, said: "I'd like to say sorry to all our passengers whose journeys are disrupted by this incident.
"Our engineers will be working around the clock to get the line open as quickly as possible.
"I also want to thank our lineside neighbours for their patience while we carry out this important work and we will aim to keep disruption to a minimum."
A rail replacement bus service is in place between Barking and Walthamstow Central until further notice.