Just after midnight tonight, the Wellington-Auckland express train will stop just short of the southern end of the rail bridge over the Whangaehu River.
The driver and train manager will walk a short distance up the track and cast a wreath of flowers from the bridge into the water below.
With the flowers will be a simple card inscribed: "In memory of all who died at Tangiwai on Christmas Eve, 1953".
The reason for the stop will be explained briefly over the train's public address system, but many of the passengers will be sleeping, just as others were at the same spot 50 years earlier.
The floral tradition was started in 1954 by the Taihape branch of the Locomotive Engineers Association to remember the 151 lost lives in what was then the world's eighth worse train disaster.
The Wellington express was carrying 285 people that Christmas Eve.
Of the 176 travelling second-class, only 28 survived.
The thoughts of passengers travelling north today will be not far removed from those of 1953.
They will be looking forward to seeing family or friends for Christmas Day, and probably carrying bags of presents.
But half a century ago, there was even more cause for excitement - the first visit of a reigning monarch to New Zealand.
Queen Elizabeth and the Duke of Edinburgh had arrived at Auckland the day before.
As express No 626 pulled out of the bustling Wellington railway station that Thursday at 3pm, some on board were probably wondering if they would catch a glimpse of the young Queen in Auckland.
Others would have been simply relieved to have got a seat on the packed train. Demand was so heavy that a second train was put on and followed an hour later.
The trip north was unremarkable - some men played cards and sipped whisky, young parents settled excited children, and Wellington workers let their minds drift to thoughts of relaxing holidays.
At Taihape, the locomotive and crew were changed.
Charlie Parker, a special grade engine driver with 33 years of service, and fireman Lance Redman replaced the Wellington crew and Locomotive Ka 949 was coupled to the express.
Further north, a southbound goods train was stopped on the other side of the Whangaehu river to give way to the express, which had priority on the main trunk line.
But as the express chugged across the tussock towards Tangiwai, a lahar - a volcanic torrent of rocks and water - was gathering speed along a nearby river bed.
Shortly after 8pm, an ash barrier on the edge of Mt Ruapehu's crater lake had collapsed, releasing a huge mass of water down the mountain into a channel feeding the Whangaehu river.
The lahar reached Tangiwai about 10.15pm in a dense wave of water, sand and boulders.
It slammed into the 60m railway bridge, dislodging two of the six piers, and rolled on to destroy three smaller bridges downstream.
When the train reached the Tangiwai bridge at 10.21pm, the girders sagged as the locomotive ran on to the drooping track.
The 145-ton locomotive, dragging a laden oil tender, arched and hit the opposite bank.
The five carriages behind were flung into the river.
The force of the impact splintered coachwork and threw many of the passengers out into the icy cold waters.
Others were trapped in the sunken carriages and were smothered in minutes by thick silt.
One survivor said he would never forget their screams.
"The cries died gradually as the water rose in the cars."
The lucky passengers travelling first class at the rear of the train felt only a jolt as the train stopped.
Among them were 15-year-old John Mahy, now of Hamilton, and his 17-year old sister, Bev.
"It didn't really feel like anything, just like the train was shunting."
An hour before, they had been sitting in a second-class carriage immediately behind the locomotive.
But a guard noticed they were carrying first-class tickets and found them two seats in the back carriages, well clear of the danger no one knew they were facing.
Mahy later saw photos of the carriage they had left, stripped to its framework and completely wrecked.
The teenagers and other passengers waited on the train for 10 hours until they were bused to Taihape.
The young Mahys did not get home to their parents in Whakatane until Boxing Day.
Those in the following train were also caught up in the mayhem.
The train picked up some survivors, reversed down the line, and diverted via the west coast back to Auckland
One passenger, Mary Arnold, said she and a friend had been caught by an airline strike and were expecting to instead travel on the 3pm train.
She telegraphed her parents, but ended up on the later train and her parents spent Christmas Day in tears not knowing until late that night that their daughter was safe.
But for those travelling in the mainly second-class carriages at the front of the train, some slowly nodding off to the rocking of the train, it was a far different story.
Suddenly, violently and mercilessly the locomotive and trailing carriages were hurled through the air after the train hit the weakened bridge, which shuddered and collapsed into the flooded river.
A sixth carriage, the first of the first-class cars, teetered on the edge of the jagged gap before it too slid into the water.
In the dark, 27-year-old Taihape postman Cyril Ellis was one of the first on the scene.
With rail guard William Inglis, he leapt into the tottering carriage, which was balanced at a 45-degree angle on the sheared-off rails, and remained inside as it toppled into the river.
When it came to rest on the bank below Ellis kicked out a window and shoved people to safety.
He was helped by passenger John Holman, who had to bite a man blocking the way to get him moving.
The lahar was at its peak when it hit the bridge but 10 to 15 minutes later, the river level fell rapidly.
Most of the dead were drowned or asphyxiated by silt. Many had head injuries.
But there were miracles too.
Four card players in the first carriage met on the riverbank, incredulous at their survival.
Nearly 300m from the bridge, a woman was found buried up to her neck in silt, still alive.
By 11.30pm an Army generator was running spotlights and arc torches flashed and sparked as workers cut through twisted steel.
Witnesses said steam rose from the water and oil flowed in dark streaks along what was locally known as Sulphur Creek.
One rescuer, former Auckland police detective inspector John Hughes, was a radio technician at the nearby Irirangi communications centre.
"I could still hear the rumble of massive boulders being carried down stream," he recalled.
"We were dumbstruck, overcome, could not get our heads around it ... it was eerie."
In the overcast dawn the receding river looked like a muddy estuary at low tide.
A bulldozer turned the wreckage and dug into the silt.
Volunteers combed the length of the river, slashing through thick scrub.
Some were suspended on ropes and lowered into deep river gorges to look for bodies.
Sixty were found 24km from the shattered bridge.
Some of the 20 passengers never accounted for would have been washed out to sea.
The driver and fireman died in the wreckage of Ka 949.
The news broke gradually to the outside world.
The first detailed account was broadcast about 1.30pm on Christmas Day by Prime Minister Sidney Holland, who travelled to Tangiwai.
The Queen delivered a message of sympathy in her Christmas speech.
The Herald news team arrived about midday that day. Photographer Graham Stewart recalls being woken with the news in the night by his father.
The 20-year-old alerted his bosses and set off with another photographer in an Austin A40.
In another car were two reporters. One, Allan Cole, later became editor of the Herald.
The boot of the photographers' car was packed with the bulky developing, printing and transmission gear needed to get their photos back to Auckland that day.
When they arrived in the early afternoon some survivors had returned to the scene, standing in a daze as volunteers worked up to their armpits in silt and mud.
After taking hundreds of shots the photographers persuaded a pilot who had landed in a nearby paddock to fly some of their film to Auckland.
The pilot made it despite strong headwinds and never having been to Auckland before.
Stewart also found a farmer whose son had a dark room, where he processed negatives and prints.
Late in the day the news team learned of Cyril Ellis' heroic actions.
Stewart and Cole crossed the bridge by foot, walking on planks laid across what was being referred to as "the gap", hailed a taxi and found Ellis at home at 9pm.
They got the story and a portrait before racing back to the farm darkroom, then to the Ohakune post office, where they filed within minutes of the midnight deadline to get their front page scoop.
Meanwhile the news had been spreading overseas.
In Johannesburg, New Zealand cricketer Bob Blair, 21, was told his 19-year-old fiancee, Nerissa Love of Wellington, had perished in the crash.
The All Blacks, also touring, toasted with sadness the "folks at home" at their Christmas dinner in a London hotel.
On New Year's Eve the 21 unidentified victims were buried, their coffins laid side by side in the Karori Lawn Cemetery at a service attended by the Duke of Edinburgh.
An inquiry - which was questioned in later years - cleared the Railways Department of any fault.
It found that railway engineers could not be held responsible for not taking steps to strengthen the bridge on the evidence of previous floodings or not predicting the strength of the lahar.
It was not the river's first lahar. A flood in 1859 carried away a bridge over the river, and in 1925 a sudden break in the crater wall sent a huge wave sweeping down the river.
The Whangaehu bridge held and reinforcing and repair work was done, although independent engineers later criticised the way the fourth pier was not embedded and left like "a loose tooth" in the river bed.
There were also accusations that warnings about the rising crater lake level were ignored.
In the inquiry, consulting engineer W. G Morrison described Whangaehu as a "dirty, evil river".
"I have crossed it many times and I have never seen it clear. There is something funny about it, and I could never treat as an ordinary river."