KiwiRail has removed 26 trains from service in Wellington after a part fell on the track causing a derailment near the capital, injuring four passengers and causing major disruption.
A rear carriage of the 7.43am train from Porirua jumped off the track just before 8.15am near the Kaiwharawhara depot on Monday, blocking lines into the Wellington station and delaying hundreds of commuters.
About 350 people were on the train at the time and four people received minor injuries, though none required hospital treatment.
A large steel air reservoir tank fell from underneath one of the front carriages and got wedged in the track. It then punched a hole through the floor of the rear carriage, forcing it off the rails.
KiwiRail, which is responsible for maintenance of the Ganz Mavag fleet, believes a weld problem might have been to blame and the 26 trains are being inspected to ensure the fault isn't widespread.
General manager of passengers, Deborah Hume, said the derailment happened opposite the Interislander ferry terminal where many rail lines cross.
Nine other services were forced to stop near the Kaiwharawhara depot and hundreds of passengers had to walk from the trains to the nearest platform.
Ms Hume told Radio New Zealand's Checkpoint programme that the welding on the undercarriages of the other Ganz Mavag units were being checked on Monday night. She said the derailed unit was checked in April this year, but could not confirm exactly what that inspection involved.
Parts of its undercarriage were dragged free from the train shortly after 2pm using a metal chain attached to a digger. About 50 KiwiRail staff also removed debris stuck in the tracks using steel cutters and hammers which spread about 250 metres behind the unit.
Engineers replaced the last of the switch rails where the network converges and the first train was able to leave Wellington station just after 3pm.
The Greater Wellington Regional Council owns the 30-year-old Ganz Mavag trains and said they are being replaced.
Manager of rail operations Angus Gabara told Checkpoint that 15 have been retired so far and the council is negotiating a contract for 35 new Matangi trains. It hoped to have these in service by mid to late 2016.
Mr Gabara said the council would wait for the results of three investigations into Monday's derailment before deciding if any changes to maintenance and inspections are needed.
Passengers worried for safety
Zoey Glasson was on the derailed carriage and told Checkpoint it was a frightening experience as people were worried it was going to tip right over.
"All of a sudden it just started right jumping up and down and people were thrown around a little bit. Some of the kids in the middle of the train started screaming - they just grabbed hold of the pole.
"I think a lot of people were scared it was going to tip over and then all of a sudden it came to a stop."
Ms Glasson said when the train stopped, she saw a big hole where part of its undercarriage had punched through the floor.
The Melling line was closed on Monday. All services have been replaced by buses and passengers were warned to expect significant delays.
The Kapiti and Hutt Valley services were running half hourly instead of every 15 minutes.
KiwiRail expects all services to return to normal on Tuesday morning.