Two investigations are underway into a derailment that left southbound lanes of State Highway 1 in Waikato blocked by containers from freight wagons early on Tuesday.
The wagons came off a northbound freight train north of Mercer at 3.20am on Tuesday, blocking both the rail line and part of the highway.
KiwiRail says the derailment also delayed other freight trains and the Northern Explorer passenger service, with trains backed up on both sides.
The traffic lanes reopened on Tuesday afternoon and general manager of freight operations, Aaron Temperton, said KiwiRail hoped to restore rail services by Tuesday night.
Mr Temperton says the train river has been stood down and an investigation is underway, entailing a detailed analysis of the track and each of the wagons to see if the derailment was caused by a mechanical fault.
The Transport Accident Investigation Commission is also investigating, with a specialist team sent from Wellington.
Nine fire crews and specialist equipment were called in and firefighters wearing breathing apparatus opened up the containers to check whether the contents were hazardous.
However, the Fire Service says the contents were not dangerous and KiwiRail then took up the salvage operation.
Two cranes removed the containers; with all traffic diverted through northbound lanes during the operation.