The line between Haywards Heath and Lewes is expected to reopen on 1 September.
Network Rail and Greater Thameslink Railway (GTR) said a complex programme of work had taken place to move the train and assess damage to the track and embankment so repairs could begin.
Temporary foundations and platforms for heavy machinery were built and specialist cranes brought in.
Rigorous tests and inspections are planned before the line reopens.
Lucy McAuliffe, Network Rail’s Sussex route director, said the location of the derailment near the River Ouse meant reinstatement of the site was “particularly complex”.
She said: “The recovery is being carried out with careful consideration for the area around the derailment site, ensuring the area’s ecology is protected and being mindful of residents living nearby.”
John Whitehurst, GTR chief executive, said: “Getting the line open in time for the end of the school holidays is an absolute priority for us, and we are working closely with Network Rail to achieve this despite very challenging circumstances.”
After the preliminary report, McAuliffe and Whitehurst said in a joint statement Britain’s railway was “one of the safest in the world”.
They said it was essential to understand what happened and identify lessons that could be learned.
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