Old wooden sleepers were to be replaced by heavier duty concrete ones.
Some of the concrete sleepers have yet to be installed.
The cause of a derailment that shut down the northern railway line last month is still unknown, according to Kiwirail.
Derailment of a freight train between Kaiwaka and Topuni on March 29 closed the line just three months after a major upgrade.
The line was closed for eight days and services resumed at 6.30am on April 6, in time for the first scheduled service after Easter weekend.
Kiwirail says its investigations indicate that the derailment was caused by a “track fault”, which has been corrected, but it could not elaborate further.
Group manager Alastair Cumming says investigations are ongoing.
“While initial inspections revealed a track fault that has been corrected, the investigation into the underlying cause of the derailment is ongoing.
“The normal timeframe for investigations of this type is around a month.”
However, a resident whose property backs on to the line believes they might have a clue.
They sent photos to Mahurangi Matters of yet to be installed concrete sleepers that have sat next to the line since it reopened in mid-January.
The sleepers were to be installed as part of an upgrade to allow heavier trains to transport cargo between Auckland and Whangarei. The upgrade was partly funded by a $200 million taxpayer contribution.
Kiwirail has twice been contacted about whether old sleepers contributed to the derailment. It said it would be “premature to speculate on the cause”.