KiwiRail says a significant section of the earthquake-damaged Main North Line has been reinstated, with the track from Picton to Grassmere now repaired.
It also says designs for six bridges along the route, which was hit by the devastating Kaikoura quake in November, are near completion.
Network services general manager Todd Moyle says more than 100 staff, consultants and contractors had worked up to and through the holiday period.
They were involved in fixing twisted track, inspecting and redesigning replacement bridges and tunnels, and getting the first part of the line, from Picton south, workable again.
"A key strategic goal was to get the line to Grassmere completed," Mr Moyle said.
"While there are no major slips in that area, there were more than 50 medium-scale faults on the line and the track had been shifted sideways in many sections along the network."
He said work began two-and-a-half weeks before Christmas, "so to have it finished by the second week of January is a remarkable feat".
KiwiRail plans to do a test run from Blenheim to Grassmere on Monday.
Mr Moyle also said designing a new bridge normally took several months, but the design team already had six ready to go so construction could begin in early February.
"There is continuing seismic activity in the area and so the safety of our work crews is our number one priority," he said.
"That makes a final completion date difficult to commit to but there is no doubt that, despite the size of the project, we are working to schedule."