The prime minister has confirmed that the UK and Scottish government transport departments are in discussion about extending the Borders Railway.
Boris Johnson's comments came in reply to a question from the Berwickshire MP John Lamont.
He raised the issue at prime minister's questions, saying it would improve transport connectivity in the Borders.
Mr Johnson said the possible extension was the subject of discussion between the UK and Scottish government bodies.
The Borders Railway - which opened in 2015 - runs from Edinburgh to Tweedbank.
There have been repeated calls to extend it through Hawick and Newcastleton and on to Carlisle.
Last year the Borderlands Partnership - made up of councils across the Scotland-England border - described it as the "best solution" to the area's transport connectivity issues.
However, it warned that work would have to start now in order to deliver the extension within the next 10 to 20 years.
A feasibility study into the project is part of the multi-million pound Borderlands Growth Deal for southern Scotland and northern England - funded by both governments.
Mr Johnson was asked to confirm support for the study as well as improvements to the A1.
He said the two government bodies were "working on" the possible extension of the line to Carlisle.
He added that a decision on the A1 would be made later this year.