Auckland's commuter rail services were due to be fully up and running on Monday morning after three weeks of disruption due to upgrades for electrification.
KiwiRail says much of the work involved lowering tracks or replacing bridges to create enough space for overhead electric cables.
It says a major part of the upgrade was in the Purewa tunnel, where train tracks were lowered by removing layers of concrete.
KiwiRail says further work is needed, such as upgrades to bridges, before electric trains are introduced in early 2014.
