A Curve to Conquer the Mountain
In the early 1900s, railway engineers faced a steep problem in the Swiss Alps near Brusio. To connect St. Moritz with Tirano, the Bernina Railway had to descend quickly through rugged terrain while keeping the gradient within 7 percent. Instead of tunnels or switchbacks, they designed a stone spiral viaduct that would turn trains in a complete circle while gradually changing altitude.
Building the Loop
The Brusio Spiral Viaduct was not part of the original plan. Engineers had first proposed a spiral tunnel, but the local geology made boring unsafe. Their solution was to build a 360-degree curve on the open landscape. The nine-arched stone structure was designed with a radius of 50 to 70 meters and a length of 110 meters. Each arch spans 10 meters. The structure rises smoothly from the valley floor, allowing trains to gain height without exceeding the steepness limit.
Opening Day
The viaduct was completed and opened on 1 July 1908, alongside the Tirano–Poschiavo section of the Bernina Railway. The railway was built to handle both passengers and freight without the need for rack-and-pinion mechanisms, which would have limited heavy loads. Local stone was used for construction wherever possible, blending the viaduct into the Alpine surroundings.
A Lasting Landmark
In 1943, the Bernina Railway was absorbed by the Rhaetian Railway, which continues to operate trains over the spiral today. The structure carries both everyday traffic and scenic tourist trains. In 2008, it became part of the UNESCO World Heritage-listed Rhaetian Railway. The viaduct remains an architectural highlight of Swiss railway engineering, looping gracefully through the mountains more than a century after it opened.
High in the Swiss Alps, a train seems to chase its own tail, looping gracefully through the mountains.
This striking sight hides a clever solution from 1908, when engineers near Brusio built a perfect stone spiral to tame a slope that once threatened to halt the line… 🧵👇 pic.twitter.com/thYH8Qybt9
— Fascinating True Stories (@FascinatingTrue) October 17, 2025