A man broke his leg jumping from a runaway dump truck on the rail line between Westmere and Whanganui yesterday.

"There were some people working on the railway line and they had a little dump truck that runs on the tracks," Whanganui senior fire station officer Bryce Coneybeer said.

"It had a malfunction and it ended up being a runaway. It was running down the hill from Brunswick heading towards town.

"The operator had to hop clear of it and let it go."

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Mr Coneybeer said the man snapped his femur while jumping off the vehicle.

Before jumping he was able to radio other workers to warn them it was coming their way, and they were able to leave another vehicle in its path to stop it. Mr Coneybeer said they could not get an ambulance up the tracks to where he was, so emergency services walked about 1.5km up the tracks with a stretcher and stabilised him.

A helicopter was called but rescuers then came up with an idea to suspend the man's stretcher from a digger, and take the digger down the tracks to the ambulance.

It was a "fairly long, slow ride back to the ambulance".

Mr Coneybeer said the truck travelled about 3km after its brakes failed, and the man had jumped out after about 2km.

A Kiwirail spokesperson said Kiwirail and one of its contractors were investigating the mechanical or brake failure.

"Worksafe has been notified and in regards to whether similar incidents have happened before, this will be part of the investigation into today's incident," they said.