KiwiRail is investigating after two carriages of the Te Huia train separated from the rest while travelling between Papakura and Hamilton.
A person onboard told Newshub half of the train had been left behind and the conductor was checking tickets in the carriage "unaware" of the separation.
KiwiRail acting chief operating officer Walter Rushbrook told Newshub the train was carrying 56 passengers plus three crew when the incident occurred.
"Between Papakura and Paerata the last two carriages detached from the front two carriages and locomotive," he said in a statement.
"Automatic braking stopped the carriages and the front of the train a short distance apart. A train attendant was in the separated carriages.
"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."
As a result of the incident, Tuesday's 5:46am service from Hamilton has been cancelled. The 6:25pm Te Huia services from Papakura will be replaced by buses.