An investigation is underway after two carriages broke away from the main train on a KiwiRail commuter service between Auckland and Hamilton.
The Te Huia service was between Paerata heading toward Pukekohe in an 80km/h zone on Monday evening when without warning it pulled apart - causing the emergency brakes to lock on. No one was hurt.
KiwiRail's acting chief operating officer Walter Rushbrook said the rear two carriages drifted apart from the front part of the train.
"When that occurs all the emergency systems kick into place and the train automatically stops."
He said a worker connects the carriages using a coupling system and it is checked before each train journey.
Rushbrook said it was a very rare event for carriages to come adrift and an engineering team is investigating what happened.
"The investigation will be looking at the couplings, the train handling, the track conditions, the train also has a black box on board so we'll find out information about the characteristics of the train right at the point of the parting."
In this case the train came to a halt, the couplings and carriages were then reconnected and checked and the passengers were moved into the front part of the train.
Rushbrook said the train with the passengers then went to Pukekohe at a slow speed of 20km/h. It was then taken out of service with the passengers continuing their journeys on buses or other trains.
Kiley O'Meara was on board with her teenage children when the carriage they were in came loose.
She said it happened at a level crossing about 10 minutes into the trip.
"So we went across a level crossing and then it just started to make this god awful screeching noise and it almost sounded steel on steel but it was obviously the emergency brakes coming on.
"Basically the back two carriages separated from the engine and the front two carriages."
The Auckland to Hamilton KiwiRail service is expected to resume tomorrow.
