KiwiRail says Wellington's passenger train fleet could be back to its full complement later on Wednesday.
Five of the new two-car Matangi units were pulled from service on Tuesday after an electrical fault was discovered in one train. A similar fault in the auxiliary power system of another Matangi train was discovered a few weeks ago.
KiwiRail says the trains, which are the first of 48 new two-car units that will eventually run on the lines, have been undergoing safety checks.
Passenger general manager Deborah Hume says they could be back in service on Wednesday afternoon.
Ms Hume says despite having fewer trains on the rail network, the peak-time service was running relatively smoothly.
The Rail and Maritime Transport Union says it is normal for new trains to have teething problems.