Hundreds of metres of track damaged in a fatal derailment near Stonehaven on 12 August is being replaced by engineers.
The driver, conductor and a passenger died when the Aberdeen to Glasgow service crashed after heavy rain.
An interim report said the train hit washed-out rocks and gravel.
Network Rail said the laying of more than 500m (546 yards) of replacement track would continue into next month as part of a "complex" repair operation.
About 400m of telecoms cables will also be put back into place along with "extensive" improvements to drainage systems.
The work follows repairs to a bridge - including 70m (76 yards) of the parapet - and an embankment.
ScotRail is operating a shuttle service between Aberdeen and Stonehaven and between Dundee and Montrose while the line is closed.
A replacement bus service also remains in place between Dundee and Stonehaven, and between Dundee and Aberdeen.
The derailment, which led to the deaths of driver Brett McCullough, 45, conductor Donald Dinnie, 58, and passenger Christopher Stuchbury, 62, is being investigated by the Rail Accident Investigation Branch.