A steam train operator's staff and volunteers are "shocked and devastated" following the death of a person at a railway station in Waikato on Sunday night.

Emergency services rushed to the station in Te Awamutu around 6.20pm on Sunday. Police confirmed one person had died at the scene.

A man on the train, who did not want to be named, said staff told them there'd been an accident.

He said people on the platform were yelling that someone was trapped under the train.

Glenbrook Vintage Railway operates the heritage rail service.

"We are shocked and devastated by the death of a person following an incident at Te Awamutu involving a Glenbrook Vintage Railway train," Glenn Deed, chairperson of Glenbrook Vintage Railway Charitable Trust Board, said in a statement on Monday night.

"Our thoughts are first and foremost with the whānau and friends of the person who died. We are providing support to our staff and volunteers and remain in contact with our passengers who were onboard the train.

"The wellbeing of everyone affected is our immediate priority at this difficult time."

The train operator was encouraging those affected to reach out to Victim Support if they needed help.

"We are deeply grateful to the emergency services that responded, to our people who cared for our passengers while alternative transport was arranged, and to our passengers for their patience and support in extremely difficult circumstances," Deed said.

A passenger told RNZ there were hundreds on board Sunday's service, which was travelling from Glenbrook, east of Pukekohe, to National Park and back.

"We weren't told what happened, but looking at the trauma of the bystanders... the trauma of staff, something I'll never forget."

She said she heard people yelling "stop" as the train left the station before it came to a halt.

"It was seconds only."

The staff were extraordinary, she said, keeping passengers hydrated and informed as much as possible.

"All while dealing with the unthinkable. They and the firemen were exemplary."

The steam train was returned to Auckland on Monday morning.

"We are cooperating fully with Police and the relevant agencies as they investigate the circumstances of this tragic incident," Deed said.

A spokesperson for the Transport Accident Investigation Commission said it was notified of the accident on Sunday. The incident has also been reported to the NZTA.