Mr Purvis criticised the gap between design and construction - (image: artist impression of reopened line)
An MSP has raised concerns about a "gaping three-year delay" in plans to reopen the Waverley line between Edinburgh and the Borders.
Lib Dem Jeremy Purvis claimed the gap would arise between design work this year and construction work in 2011.
He called for "more answers" on the timetable for the £295m scheme.
A Scottish Government spokesman said it wanted to ensure best value for the public purse but added work would be under way by 2011 "at the very latest".
Mr Purvis received information about the plans for reopening the line in a parliamentary answer.
"The SNP are planning a gaping three-year delay to the project, with the construction phase of the project not expected to start until 2011, despite the target date of the design stage being later this year," he said.
"If, as the minister has said, banks are queuing up to lend the money to build it, why do Borderers have to wait another three years for the rail link construction to begin?
"More answers have to be given as to why there is this massive gap of three years between the end of the design and the beginning of the construction of the Borders railway."
He said construction work on the project had originally been expected to start in 2009.
A Scottish Government spokesman said that ministers were committed to progressing work on the Borders railway as quickly as possible.
"That means starting in early 2011 at the very latest," he said.
"Everyone will understand that with any major transport project of this scale, a thorough procurement process, including tendering, clearly needs to be undertaken before construction can begin.
"The length of this will depend on the market and negotiations with bidders as we need to ensure that we achieve the best value for the public purse, whilst ensuring that a strong management structure is in place."
Earlier this month Transport Minister Stewart Stevenson confirmed the costs of the project had more than doubled and the completion date had slipped from 2011 to 2013.
He also announced that a non-profit making company would be set up to pay for the bulk of the costs.