Trains will resume running on the Main North Line between Picton and Christchurch in stages from Thursday.
But KiwiRail fears motorists may get caught out at level crossings, after the absence of trains since the devastating November 14 Kaikoura earthquake.
Widespread damage occurred on the lines.
KiwiRail Group General Manager Network Services Todd Moyle said motorists needed to be vigilant around level crossings.
“It means people need to expect trains anytime and from either direction.”
There were still 750 damaged sites, he said.
The first work train is scheduled to run from Blenheim south to Mirza, near Ward, then progressively further south.
At the end of the month trains will start running from Christchurch, northwards.
As the recovery works proceed, the frequency of track machinery travelling on the line will also increase.
“The trains are a vital part of the re-build programme,” said Mr Moyle.
“The trains are being used to carry ballast and other materials to work sites on the line, and are a vital part of rebuilding plans,” says Mr Moyle.
TrackSAFE Foundation Manager Megan Drayton said pedestrians also needed to be alert.
“People need to understand they must stay off railway tracks, and cross only at level crossings.