Investigators say a carriage was dragged across sleepers and ballast for about 1.5 miles (2.4km).
Trains are running again along the route where a train partially came off the tracks on 14 August.
Train passengers in Essex are being warned of slower speeds and timetable changes - what's going on?
Greater Anglia says repair work has started after the train was removed from the tracks.
A railway line to Southend-on-Sea remains closed after a train came off the tracks on Friday.
The train came off the tracks on Friday, with the line expected to remain closed for several days
Greater Anglia says there is extensive damage along 1.5 miles of track after Friday's derailment.
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Greater Anglia says it is working "around the clock" the resume normal services as soon as possible.
