A total of 44 per cent of work on the 30-mile line between Edinburgh and Tweedbank, south of Galashiels, has been finished.
Network Rail said it expected to pass the half way stage by the end of the month.
Trains are due to run again in September next year for the first time in 46 years since the closure of the Waverley line to Carlisle.
Construction of the £350 million scheme, which started a year ago, is due to be finished next summer, which will be followed by line testing and driver training.
Network Rail, which is responsible for the £294m construction contract, said 866 staff had clocked up 2.1 million hours on the project so far.
More than 230 workers from supply firms have also been involved and caterers have supplied 25,000 bacon rolls to construction crews.
Construction started in April last year at Shawfair in Midlothian, and has included stabilising old mines, restoration of the Lothianbridge viaduct and two tunnels, and re-routing a section of the Edinburgh City Bypass during the building of a bridge to carry it over the line.
The work has also involved installing 16 miles of drainage, refurbishing other 104 bridges and moving 804,000 tonnes of earth - enough to fill at least five Olympic size swimming pools.
Transport Minister Keith Brown, who had hoped the project would be finished this year following significant delays and cost increases, said: “Network Rail and [principal contractor] BAM should be applauded for the progress they have made to get us to this stage.
“That we will soon see this service re-introduced is a huge achievement for all the partners involved.
“It is an extremely exciting prospect for those people living all up and down the route who will feel the benefits in terms of their increased access to jobs and social opportunities, as well as the boost to tourism and investment and likely regeneration in these areas.”
Network Rail project director Hugh Wark said: “The entire team has done a fantastic job in getting to this stage. By the end of this summer, the bulk of the structures and earthworks will be complete, and we’ll be looking forward to laying the railway track along the route.”
David Spaven, rail consultant and author of Waverley Route - The Life, Death and Rebirth of the Borders Railway, said
“It’s great to see the engineers making such good progress, but tragic that Transport Scotland have cut back the specification, with 40 per cent less double track than promised four years ago, and no ‘future-proofing’ of the structures south of Gorebridge to allow cost-effective double tracking of the single-track sections.
“This is a penny-wise pound-foolish approach to what should be a strategic addition to Scotland’s rail network.”