In building a live steam railroad, we are are always careful to ensure that our radius is not too tight and our grades are kept to a minimum. What do you do to keep your grads at a minimum if your a mountain railroad? It takes imagination and planning. That’s just what they did in Graubunden, Switzerland. This is a section of the Bernini Railway.
“A spiral viaduct was required immediately south of Brusio to limit the railway’s grade to the required maximum of 7%, so the train would not slip on the way up, or be uncontrollable on the way down. Geological factors prevented the boring of the spiral tunnel that had originally been planned. Engineers therefore decided to construct on the valley floor a 360 degree curve with a 50 to 70 m (160 to 230 ft) radius, and the viaduct forms part of that curve.”