KiwiRail is hoping to free some rail wagons next week that had been trapped near the Grassmere saltworks in Marlborough by the November 14 earthquake.
This followed reinstatement of part of the quake-stricken route, from Picton to Grassmere.
KiwiRail spokesperson Todd Moyle said more than 100 KiwiRail staff, consultants and contractors had been working on the project.
In addition, design work had been completed for six new bridges - which Mr Moyle said would normally have taken months.
The government announced just before Christmas that the Picton-Christchurch rail link would be restored, along with State Highway One.
It was estimated at the time that the cost would be between $1.4 billion and $2 billion, and it was hoped to have the work completed by the end of this year.
The cost would be shared between KiwiRail and the New Zealand Transport Agency.
Mr Moyle said the Transport Agency's share would be greater than KiwiRail's, but he did not give any proportions.
He added some of KiwiRail's cost would be covered by insurance, but again did not reveal any numbers.
The latest work was part of KiwiRail's contribution to restoration of the link.
Mr Moyle said the scale of the work was huge, especially in the badly damaged area between Clarence north of Kaikōura and Oaro to the south.
At present it was not clear what is behind the slips that are blocking the line, and it was important to make sure these were stable so that a repaired line would be resilient.
"It's probably the biggest project we have had in the South Island since the main northern line was built in the 1950s," he said.
An extra problem stemmed from continuing aftershocks in the region.
"Every time there is another shock you have got to reassess and make sure you are doing the right work, so it is difficult to give a precise time frame for the entire project at this stage."