The rail line between Stonehaven and Montrose will be closed until at least 7 February after a bridge wall collapsed, Network Rail has announced.
The 24m (79ft) section of the side wall broke on 15 January and is located about a mile north of where three people died in last year's fatal derailment.
Network Rail Scotland said the rural site was a "complex one".
A further update on reopening timescales is expected next week.
Replacement bus services are in operation.
A statement from Network Rail Scotland said assessing the condition of the bridge was complicated "due to the rural area, exposed location and height (15m)".
It added: "So far we've completed extensive surveys and site inspections, including removing the southbound track and its supporting stone.
"We're carrying out extensive checks on the northbound side too in the coming days.
"This will help us confirm the full extent of the repairs needed and set timescales for reopening the bridge for customers."
The line had 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.