The crash could have caused a major disaster as a train was just minutes away when the van landed on the tracks.
The van left State Highway 73 between Cass and Bealey Spur, south of Arthur's Pass, in thick fog about 6am and rolled down the steep bluff.
The van did not have any back seats and Ms Wood was thrown from the vehicle, according to a source connected to the investigation, who did not want to be identified.
"It's always dodgy driving with nobody strapped in. But we haven't determined whether a lack of seatbelts was a major cause yet," the source said.
Police were awaiting the results of toxicology tests to see whether alcohol was involved, and also whether fatigue was a factor.
A train had been scheduled to pass through the area within minutes of the crash.
"There's trains scheduled to pass through that area all the time. In the time we were there we had to hold up four or five of them," the source said.
"Had it been cleaned up by a train it could have been [a major disaster]. That's the main rail feed to the West Coast so it's a busy line."
A KiwiRail spokeswoman said about 130 passengers travelling from Christchurch to Greymouth had to be transferred onto buses as a result of the crash.
Westpac Rescue Helicopter pilot Quentin Hulse said there was thick fog covering the site so they had to land about 500m away and walk to the scene.
"The van was really munted, I supposed is the best way of putting it, and lying on the railway line with quite literally bodies scattered everywhere with various injuries.
"It was pretty bad."
Barrie Drummond, a former railway worker who lives 4km from the crash site, arrived there about 7.30am. He said the vehicle was "squashed like a tin can".
"It couldn't have been a worse morning for fog and it was lucky that a coal train coming from Westport was stopped about 10km from Arthur's Pass or it would have been a terrible disaster," Mr Drummond told the Herald on Sunday.
He was told the van's driver had somehow scrambled up the bank through the bush and managed to stop a passing milk tanker to help.
"I believe that the woman who died was found in the river that runs alongside the railway lines," Mr Drummond said.
Ms Wood was a past student of Greymouth High School but had recently moved to Christchurch, where she was a "full-time mum" according to her Facebook page.
The page named In Loving Memory of Faith Wood was set up yesterday afternoon and had received 547 'likes' by 7.30pm.
The unnamed person who started the page wrote: "Faith, i remember at Grey Main School, you stood up to the bullies for me, you made me realise that you get bullies everywhere, you taught me how to handle them, how to be strong, because of you, im stronger than ever."
Other friends posted photos of Ms Wood and notes of remembrance. "I wish we cud rewind tym and take it bk to friday and me not let u go to chch i miss you hun," wrote Olivia Batchelor.
"That amazing smile and beautiful heart of yours, you were one in a million, you'll be missed and most definitely never forgotten. love you til the day i die, see you when im looking at you.. fly high mumma!! We'll watch over Sas," posted Latesha Wade.
Ms Wood was the first of two women to die in separate crashes on the same alpine highway yesterday.
Tracy Ann Shipston, 39, also of Christchurch, died instantly when her vehicle struck a tour bus head-on near Porters Pass in the foothills of the Southern Alps about 5.30pm.
There were no passengers in the tour bus at the time, and a St John Ambulance spokesperson said the bus driver received minor to moderate injuries.
Police said weather and road conditions were fine and it was unclear what had caused the collision.
Initial indications were that the car had crossed the centre line into the path of the oncoming bus.
- additional reporting: Russell Blackstock, John Weekes and Calida Smylie
