Repairs will now keep the line between Montrose and Stonehaven closed until at least Sunday.
Trains were halted after ScotRail staff spotted an 80ft (24m) long section of the river bridge’s side wall had fallen away, about three miles west of Stonehaven.
It happened on the Carron Water Bridge, about one mile north of where a ScotRail train hit a landslide at Carmont after heavy rain in August, in which driver Brett McCullough, conductor Donald Dinnie and a passenger, Christopher Stuchbury, died.
Network Rail Scotland said the bridge was last inspected in October as part of preparations to reopen the line after the derailment.
It said there was no significant change in the 180-year-old bridge’s condition since it received a detailed inspection in 2018.
The cause of the collapse is still being investigated.
A Network Rail spokesperson said: “There was no significant deterioration in structural condition noted in these inspections.
“No major work has been carried out on the bridge’s side walls since the installation of a metal handrail on the parapet in 2013.
“Significant work was undertaken in the river beneath the bridge in 2020 to help protect the foundations of the structure from potential water damage when the river floods.
"None of this work took place at track level.
“Structural assessments will continue over the coming days as the team on site makes plans for the bridge’s repair.
"It is too early to confirm exactly what work will take place, or timescales for its completion.”