Part of a rail bridge has collapsed near the site of the fatal Stonehaven train derailment.
A 24m (79ft) section of the side wall has fallen from the bridge, about a mile north of where three people died when a train left the track and crashed last August.
Network Rail said it was a "structural fault" and not caused by a landslip.
The line between Aberdeen and Dundee remains closed while structural engineers assess the fault.
The structure is located three miles north of Carmont signal box. The collapse was discovered just before 10:00 on Friday.
The rail company said the damage to the parapet was "extensive" and that the line was expected to be closed for a "significant" period of time while repairs to the bridge take place.
👷♂️👷♀️ We've closed the line between Aberdeen and Dundee while our engineers repair a damaged side wall on a bridge near Stonehaven. Specialist structural engineers are assessing the fault and planning repairs. We'll work around the clock to complete repairs as quickly as possible.— Network Rail Scotland (@NetworkRailSCOT) January 15, 2021
The Network Rail Twitter account told followers engineers would be working around the clock to complete repairs.
Specialist staff are also checking similar bridges as a precaution.
The line between Aberdeen and Dundee had just reopened in November, nearly three months after the Stonehaven derailment.
The driver, a conductor and a passenger died when the Aberdeen to Glasgow service derailed near Stonehaven on 12 August after heavy rain.
An interim report said the train hit washed-out rocks and gravel.
A Network Rail spokesman said: "The line is currently closed while our engineers repair a damaged side wall on a bridge between Carmont and Stonehaven.
"Specialist structural engineers are currently assessing the fault and putting plans in place for its repair.
"Our engineers will be working around-the-clock to complete this work as quickly as possible."