What you're travelling through took nearly 20 years to build. Construction began in 1923, and the line didn't fully open until 1940. For close to two decades, this valley was a workplace for up to 280 men at a time.
The tunnels were the challenge. Of the twenty on this line, eighteen were carved entirely by hand - workers drilling into rock face, packing dynamite, retreating, blasting, clearing rubble, then doing it again. Progress averaged one metre per month. The Nåli tunnel alone took eleven years.
One tunnel ahead makes a complete 180-degree turn inside the mountain. The technique came from Switzerland. Robert Gerwig had pioneered spiral tunnels on his Schwarzwaldbahn in the 1870s, then applied them to the Gotthard Railway's famous loops at Wassen. Norwegian engineers studied these methods - many at ETH Zürich - and brought them home.
But how do you keep a curving tunnel aligned when you can't see daylight? With a theodolite - a precision instrument for measuring angles - and mathematics. Surveyors broke the curve into short straight segments, measuring tiny deflection angles at each station by lamplight, trusting their calculations would bring both ends together.
The workers lived in barracks along the route - sixteen men to four rooms. They paid for their own cook, blacksmith, and dynamite.
Local farmers competed fiercely for transport work, hauling materials by horse at night and tourists by day. The competition led to actual fights until traffic police were brought in.