One insider said there was “not a hope in hell” the line would be completed by the end of 2014 as planned – and trains would now not run until 2015 at the earliest.
Construction work on the Edinburgh-Tweedbank line is not expected to start until at least this autumn – a year late.
This came despite transport minister Keith Brown insisting the project was on course when he viewed preliminary work in Galashiels yesterday.
Mr Brown announced five months ago that Network Rail would take over the scheme after novel plans for the private sector to run it were scrapped. That came after two of the three shortlisted bidders pulled out. However, Network Rail is still assessing the 30-mile route to see if it can be finished on time and within the £235 million-£295m budget.
The rail firm is not expected to reach a decision until next month, to be agreed with the Scottish Government’s Transport Scotland agency in April.
Network Rail would then have to advertise for a contractor before work could begin.
Scottish Borders Council leader David Parker, who met Mr Brown yesterday, said he expected work to start in November or December and “probably be complete” in mid-2015.
He said: “The timetable is ambitious and will be tough for the contractors to meet.”
An industry source said: “There is not a chance in hell of delivering it within the current timescale. There is some way to go – it could be a year late.” The source said the project would be prone to delays from bad weather and problems with bridges and ground conditions.
Another industry source said: “Nobody in the rail sector feels it can be delivered at a sensible cost if the Scottish Government sticks to the existing timetable.”
Labour transport spokeswoman Elaine Murray said: “It would seem extraordinary that Transport Scotland can guarantee a delivery date when the programme of works has still to be finalised and a contractor has still to be appointed.”
However, Midlothian South, Tweeddale and Lauderdale Nationalist MSP Christine Grahame, who had invited Mr Brown to visit the project, said: “Progress is going ahead as expected. There is no slippage.
“The preparatory work will mean the job of track-laying is done more efficiently.”
A Network Rail spokesman said it was reviewing the project prior to taking it over. He said: “Network Rail is carrying out a due diligence exercise and review of the project prior to the transfer of Authorised Undertaker powers later this year. As part of that process we are considering the programme and milestones for the project.
“We expect to discuss the outcome of our review and our subsequent proposals for the delivery of the new line with Transport Scotland and the ORR [Office of Rail Regulation] this spring. How the new line will be delivered forms part of our review and no decision on the tendering process has yet been made. We will deliver the new line as quickly as possible and are aware of the timelines and budgets the government would like to meet.”
Last night a Transport Scotland spokeswoman said: “The advance works viewed by the transport minister will ensure the project can be built by Network Rail in the shortest possible time.
“Ministers remain committed to the target project delivery date of December 2014.”