"This will increase the cost of the project beyond the previous £742m estimate and NR's cost estimates for a number of other major projects which are at earlier stages of delivery have also increased," Yousaf said in a ministerial statement on July 5. "I am not prepared to simply accept the long-term cost implications, nor the revised programmes that NR have set out. I will also be doing everything possible to ensure that NR delivers the full [Control Period 5] programme in Scotland by March 2019 and without any extra funding from the Scottish government."

Yousaf has instructed Transport Scotland to carry out a review of NR's enhancement programme in Scotland, focussing on governance structures for the delivery of major projects. "In the case of the Edinburgh - Glasgow Improvement Programme (EGIP), there is evidence of poor management of contractors; and across the programme there are systemic issues including poor planning and cost estimation and a failure to properly incorporate well established regulations into their project plans," says Yousaf.

In its Network Rail Monitor Scotland report for October 2015-March 2016, which was published on July 5, the Office of Rail and Road (ORR) said that progress on Scottish enhancement projects has been "varied" and the regulator expresses "concerns regarding the ability of key projects to meet their obligations and regulatory milestones."

In addition to Key Output 1 (Edinburgh - Glasgow electrification), the report warns that other key Scottish infrastructure projects, including the upgrading of the Perth - Inverness Highland Main Line and modernisation of the Aberdeen - Inverness line are making slow progress.

"Systemic issues around project planning and delivery continue to present challenges for NR," the report states. "These include complying with relevant electrical safety legislation and technical specifications. We note however that Network Rail is addressing these issues though the Enhancements Improvement Plan (EIP) and we will be holding the company to account for its implementation."

Yousaf says that while the start of limited electric services will be delayed from December until next summer, the full electric service will still begin operating between Edinburgh and Glasgow with the new fleet of Hitachi class 385 trains in December 2017 as originally planned.