Improvements to the Edinburgh to Glasgow line — a move that would make the journey just 42 minutes between the cities — are now expected to cost £858m, an almost £200m increase on the original cost estimate set in 2012.
Rail regulators described the cost increase as “deeply concerning” and revealed they have launched a probe into the delays and budget overruns on the Transport Scotland electrification project.
The Office of Rail and Road said: “The ORR can confirm the latest estimated figure received from Network Rail for Edinburgh Glasgow Improvement Programme (EGIP) is £858 million.
“This increase in cost, aligned with Network Rail’s delay in delivering EGIP is deeply concerning.
“We will publish a lessons learned summary report later this year focusing on the electrification of the Edinburgh to Glasgow line and the underlying causes behind cost increases and programme delays.”
Originally scheduled to be completed by the end of last year, the scheme to link the two cities has faced a number of delays with both Network Rail, who are responsible for supplying the tracks, and ScotRail, who are running the trains.
Concerns have been raised about whether the eight-car service and 42-minute journey time will be available by December this year.