Work to replace rail tracks and sleepers inside the Dunedin tunnel began in April and are expected to continue for a few more months.
KiwiRail South Island regional infrastructure manager Gary Ikin said the tunnel was part of the Main South Line and an important part of the rail freight connection to Port Otago.
It was also used by Dunedin Railways for charter services.
Mr Ikin said the work was being done one track at a time so that freight services could continue running.
It involved replacing rail and sleepers, but KiwiRail was taking the opportunity to upgrade the formation — the rock foundations under the tracks — among other improvements.
This would ensure the tunnel and rail infrastructure remained reliable for the next 50 years.
So far the formation under one of the two tracks, the Loop Track, had been replaced.
Improvements to drainage in the tunnel and work to install new signal cables were also under way.
Over the next few weeks KiwiRail would work towards reinstating the ballast and the loop track, he said.
The next steps would include removing the other track in the tunnel, the Main South Line, to replace the formation, ballast and track.
Upgrades to the turnouts, which allow the trains to switch between tracks, near the tunnel would also take place.
