Carriages were added to suburban train services from the Kapiti Coast into Wellington on Tuesday morning after commuters complained of overcrowding on Monday.
One passenger said the train was half an hour late and commuters were packed in like sardines.
The new service extending the Paraparaumu line to Waikanae opened on Sunday as part of the $550 million upgrade of Wellington's rail network.
KiwiRail passenger manager Deborah Hume says the company did underestimate the number of people who would use the trains but the delays were mainly due to mechanical issues.
Ms Hume says the trains causing the problems will be replaced during the upgrade but warns there may be some more teething problems for the service this week.