The darkness of the 8.5km long Otira tunnel marks the dramatic transition from one side of the Southern Alps to the other.
Before the TranzAlpine train entered the tunnel we had been climbing up the ruggedly beautiful mountain country on the eastern side of the range with wild stony rivers, intimidating rock faces, slopes of heather and majestic snowy peaks.
But when we burst back into the light 15 minutes later we were going down through the equally spectacular lush green mountain forest produced by the 5-6m of rain which falls on the West Coast each year.
The tunnel is an impressive link between the South Island's two coasts having taken 15 years to build, involving 60,000 men working 24 hours a day, drilling in from either side of the mountains.
When the two ends finally met the alignment was less than 2cm out. It's said to be so straight that if you stand at one entrance you can see a pinprick of light at the other.
For its time this was an amazing piece of engineering. When it opened in 1923 it was the longest tunnel in the British Empire and the longest in the Southern Hemisphere.
As you'd imagine, this is hard country and has produced some remarkable characters and amazing stories over the years.
Soon after we boarded at Cass we passed by the Bealey Hotel, on the opposite bank of the Waimakariri River, and Scotty the train manager recalled how in the old days the hotel owner used to offer travellers a unique deal.
"If you paid him to take your car across the river with his horses he'd charge you 1 (which would probably be equal to $100 these days). But if you tried to get across on your own and got stuck he'd charge 5 to pull you out."
A later publican at the hotel, Paddy Freaney, achieved international fame in 1993 when he and two others claimed to have seen a moa in the Craigieburn Range and produced a blurred photo as evidence. The claim sparked an intensive search, which found no evidence, but these days there is a moa statue at Bealey to commemorate the story.
Probably the best known station on the line is Arthur's Pass, named after Arthur Dobson who surveyed the route in 1964. It's the gateway to the 94,500ha of wilderness in Arthur's Pass National Park and hordes of fit looking people got off here. At 737m above sea level it's the highest station in the South Island. And, just to prove that it's high in the mountains, sitting on the roof of the station was a kea with a wicked twinkle in its eyes.
There are several stories told about the tunnel's construction, including one of an amazing escape when the roof collapsed after heavy rain. One worker, James McKeich, was dragged out from the rubble but seemed to be dead. The rescuers called his son, who was working on the project as a carbide boy, saying, "Come here, son, and see your father for the last time."
But the boy noticed his father's upper set of false teeth were missing, a pair of blacksmith's tongs was thrust down McKeich's throat and used to pull the broken dental plate from where it had been wedged in his gullet, and, he recalled afterwards, "I started to breathe again and came to." Further down the Alps, on the banks of the Grey River, is a poignant piece of coal-mining history, the remains of the Brunner Mine, named for Thomas Brunner who discovered coal here in 1846. Once the country's biggest coal mines, it is mainly remembered today for being the scene of our greatest mining disaster, in 1896, when 65 workers died following a gas and coal dust explosion.
The mine closed in the 1930s and the huge complex of mines, coke ovens and brickworks was left to deteriorate until the 1970s when the Historic Places Trust intervened to protect it as a significant industrial site which you get good views of from the train.
Following the Grey River - named, ironically, after one of our most colourful governors and politicians - leads naturally enough to Greymouth and the end of our journey around the country by rail ... but, as you'd expect on the West Coast, it didn't mean an end to the stories.