The Office of Rail and Road (ORR) said the track body had failed to provide “tangible evidence” of the steps it had taken to implement action plans prompted by recommendations by two independent reports into the derailment near Stonehaven in Aberdeenshire in August 2020.
It came as Britain’s rail network was severely disrupted by record temperatures on Tuesday, with UK transport secretary Grant Shapps saying the Victorian-era infrastructure "just wasn't built to withstand this type of temperature".
The Carmont crash was caused by debris being washed onto tracks during heavy rain.
The ORR’s 2021/22 assessment of Network Rail Scotland (NRS) stated: "We commenced a programme of inspections to assess Network Rail’s response to recommendations made in the post-Carmont derailment task force reports, authored by Dame Julia Slingo and Lord Robert Mair.”
It said NRS had “described the steps they have taken in response to their action plans, but have subsequently failed to provide any tangible supporting evidence to demonstrate these”.
"NRS also did not clearly articulate how each action plan will implement the relevant taskforce recommendations,” the regulator said.
The ORR said it had improved “identification of its drainage assets”, but the checks on these were delayed by this year’s storms.
It said: “Improved information about drainage assets is vital for NR to ensure its resources are correctly targeted to maintain existing assets and invested in meeting the challenges presented by climate change.”
The ORR said there was backlog in examining structures and earthworks and NRS had missed its efficiency targets by 21 per cent.
The regulator said: “There is a potential that failure to examine earthworks and evaluate examination reports frequently enough could lead to an increased safety risk.
"This risk may be increased as earthworks are more likely to be impacted by severe weather events.”
ScotRail punctuality over the year was 90.2 per cent, 2.3 percentage points below target, but Caledonian Sleeper’s was 80.9 per cent, 0.9 percentage points above target.
NRS had the lowest overall performance “scorecard” among the five regions, of 42.6 per cent.
However, freight performance was the best across Britain and improved significantly, with growth targets met.
ORR senior regulation manager for Scotland Jennifer Cullen said: “Overall, it was a challenging year for [NRS], but we continue to work closely with it to identify areas of concern.
“In response to poor train service performance, NRS and ScotRail have already established a joint performance improvement plan including targeted interventions that, if delivered, should result in a more resilient railway.”
A Network Rail Scotland spokesperson said: “We are committed to learning the lessons of Carmont and to delivering on the recommendations made by the Rail Accident Investigation Branch, as well as implementing the Mair report.
“Since the accident, we have made significant changes to how we operate services during extreme weather and have increased our investment in new technologies.
"Our control room is the first in Britain to establish a specialist weather team, and is being supported by increased investment in our front line teams.”
“We’ve made positive steps, having delivered around £64 million of efficiencies in the last year, and we remain committed to identifying ways to further improve efficiency.”