Five million tonnes of freight was carried through the Kaimai Tunnel during the past financial year on an average 23 trains a day.
But all traffic was brought to a halt today to mark a special occasion.
Workers celebrated four decades since the official opening of the 8.9km tunnel, which runs through the Kaimai Range, on September 12, 1978.
A cake with the number 40 shaped in black icing was placed at the entrance of the tunnel for workers to celebrate the milestone.
But before the cutting of the cake, the workers removed their hard hats for a moment of silence to honour the four men who lost their lives in the 1970 disaster when the tunnel collapsed.
Among the men paying tribute was Terry Trotter, who has worked at KiwiRail for 10 of the tunnel's 40 years.
Trotter often stood at the tunnel's entrance on the Waikato side to watch the sun go up.
"Quite often the sun is rising on the Tauranga side, and there have been occasions where the climatic conditions are just right, and all you can see is what looks like a flicker of flames coming from the other end of the tunnel," he said. "That is the sun."
Trotter remembered arriving at the portal one morning to find some of the men in a panic.
"The guys thought there was a fire in the tunnel," he said. "I had to convince them it was the sun coming through the tunnel."
Throughout his career, Trotter has spent eight years as a structures inspector at the tunnel and the past two years as construction manager.
"I am quite familiar with the old girl," Trotter said.
So what is it like inside?
"It is dark, it is long, and it has got a unique smell," he said. "You always get that lingering smell of oil and diesel."
It is also sweltering - about 26C.
Peter Dix has been working at the tunnel for about 10 years as the area manager for the East Coast Main Trunk.
He said the tunnel structure was in good condition, but there had been "a few challenges" with continuous water flow through the rock building pressure inside the tunnel.
"The way the track was constructed back then it had a pad, then a 700mm of track floating and another pad fastening where the pads were," he said.
"What we have done as part of the first project is put a continuous pad down underneath the track ... securing the bottom of the track where little bits of erosion had happened over time."
Workers inject resin underneath the concrete slab and "jack it back up" to the same level the same way used to correct the foundations of the buildings damaged in the Christchurch earthquake, Dix said.
"We are doing exactly the same thing here."
New technology had now allowed workers to inject the resin deeper into the bedrock which stops any further erosion, he said.
Dix said the celebration was a special day for the tunnel and its workers.
"It is a milestone, particularly because it has such a bright future still. In a different era it may still have been its end-of-life celebration," he said.
"It is a really good story as far as how far engineering in New Zealand has come from when we first built it."
- Work began on the Kaimai railway deviation and the Kaimai Tunnel between Waikato and the Bay of Plenty on October 2, 1965.
- On February 24, 1970, during the early stages of underground work, a cave-in trapped 12 of the workers. Eight men were rescued, but four lost their lives: James Smart, Alfred Thomas Leighton, Donald Alexander McGregor and Peter James Clarkson.
- When Prime Minister Robert Muldoon formally opened the tunnel on September 12, 1978, he also unveiled two plaques set into a boulder beside the Old Te Aroha Road, near the western end of the tunnel.
- One commemorated the opening of the tunnel; the other paid tribute to the four men who had died and others who had worked to complete the tunnel.