The East Coast route, running out of London King's Cross, serves locations including Peterborough, Leeds, York, Newcastle, Berwick upon Tweed, Edinburgh's Waverleystation, Glasgow, Inverness and Aberdeen.
The new 35-mile line will run from Waverleystation in central Edinburgh as far as Galashiels and then to Tweedbank where there will be a park and ride which is designed to give access to the network to people from surrounding towns.