KiwiRail is advising passengers in the Wellington region to arrive early for their trains on Monday morning, because of changes to the timetable.
A new schedule is in place because of the new service to Waikanae, on the Kapiti Coast.
The changes came into effect on Sunday but KiwiRail says the crowds travelling to work on Monday morning will be the first big test.
Passenger manager Deborah Hume says commuters should check the new timetable and arrive at their stations a few minutes early. Trains may leave at different times and from different platforms than they did before.
KiwiRail says about $600 million has been spent on upgrades, including a new station at Waikanae.
Ms Hume says the work has disrupted services during the past few months but the bulk of it is now finished.