The railway between Lewes and Haywards Heath will be closed for at least another 10 days while a derailed train is recovered and track repairs carried out.
Three carriages of a passenger train came off the track near Lewes, in East Sussex, on Thursday, leaving two people with serious but not life-threatening injuries and another 18 taken to hospital.
The job of moving the damaged train can begin now after the Rail Accident and Investigation Branch completed its work at the site on Saturday and handed it back to Network Rail.
Network Rail and Southern Rail said the "complex" operation will see the carriages, which slid down an embankment, removed by crane.
The companies said the location made things difficult.
In a statement, they said: "The railway is close to a river, in a site of special scientific interest, and restricted road access is only via a residential area.
"As much specialist equipment as possible is being brought to the site by rail, given the challenges of access by road."
The two firms said engineers were preparing the area for two 125-tonne rail-mounted cranes - some of the biggest available for railway use.
"Before lifting can begin, teams must make sure the ground and railway are stable and strong enough to safely support the cranes and the weight of the carriages," the statement added.
Once the train has been removed, engineers will be able to fully assess the damage to the railway and "understand the scale of the repair work needed before services can resume safely", the rail firms said.
John Whitehurst, interim CEO of Greater Thameslink Railway, which runs Southern Railway, said: "We're working closely with Network Rail to do everything we can to keep people moving while recovery work continues and the railway is restored safely."
He said replacement transport would be provided where needed and extra staff would be on hand to help passengers at stations.
Network Rail's Sussex route director Lucy McAuliffe said: "We are working around the clock to get this done. We will share more updates as the recovery work progresses."
The train left the tracks in Lewes at about 15:44 BST on Thursday 13 August.
The incident occurred as the Eastbourne-bound Southern service approached the East Sussex town from London Victoria.
About 150 people were on the train - the 14:24 service from London Victoria to Eastbourne - when it derailed.
All the passengers were evacuated from the carriages with the help of the emergency services.
Passengers are being advised to check their journey before travelling and allow extra time.
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