South of Scotland Liberal Democrat MSP Jim Hume is pressing the Scottish government to commit to a feasibility study to expand the line to Hawick, and eventually south west to Carlisle.
The new section will be officially opened by The Queen on Wednesday.
Accompanied by the Duke of Edinburgh, the monarch will visit Edinburgh, Midlothian and Tweedbank for the Scottish Borders Railway opening celebrations.
Mr Hume said: “This is a historic moment for the Borders, but the Scottish Government must now acknowledge that the link to Tweedbank is the building block for real progress on an extension to Hawick and eventually Carlisle. I want Scottish Ministers to get that feasibility study underway.”