Two carriages of the Te Huia train got separated from the rest while travelling between Papakura and Hamilton.
KiwiRail acting chief operating officer Walter Rushbrook said the train was carrying 56 passengers plus three crew when the mechanical incident occurred.
Tuesday's 5.46am service from Hamilton had been cancelled and the 6.25pm Te Huia services from Papakura would be replaced by buses.
"Between Papakura and Paerata the last two carriages detached from the front two carriages and locomotive," he said in a statement.
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"Automatic braking stopped the carriages and the front of the train a short distance apart. A train attendant was in the separated carriages.
"KiwiRail apologises to the Te Huia passengers and to those affected by subsequent disruption on the Auckland network."
Rushbrook said KiwiRail was investigating the circumstances of the incident and until that investigation was complete, it would be premature to speculate on the cause.
"No-one was injured and passengers in the separated carriages were moved into the front carriages and the train has been re-connected and is continuing to Pukekohe where passengers will be transferred to continue their journey."
Delays on the network can be expected, Rushbrook said.
A passenger told Newshub half of the train had been left behind and the conductor had been checking tickets in the carriage "unaware" of the separation.