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.
"Automatic braking stopped the carriages and the front of the train a short distance apart. A train attendant was in the separated carriages.