The secret is adhesion
Why doesn't the train just slide backwards?
The trains of the Rhaetian Railway run up and down extremely steep inclines. So how can the red trains climb such steep mountains, without simply sliding back down? Why do metal wheels running on equally-smooth steel rails not just slip when tackling a mountain? The magic word is „adhesion“.
This technical term, which denotes the force that keeps the wheels of the red trains on their rails, could also be defined as „invisible glue“. If the rails are too steep, the „glue“ no longer sticks. The train might attempt to ascend, but cannot – because the force is too weak. The wheels could then begin to slip, causing the train to slide back down the mountain.
Clever: the Circular Viaduct actually flattens the incline
In order to prevent this ever happening, the Rhaetian Railway includes various items of infrastructure designed to make the rails less steep. The Circular Viaduct in Brusio, for example, leads the train down via a descending spiral. The switchback layout of the track means that it is less steep, but longer than it would be if the line went directly up the mountain. If a slope is too steep for a train to tackle, and adhesion no longer works, help is needed. Items such as toothed wheels or cables can then be used to pull the train up the mountain. The Rhaetian Railway is however designed so as not to need such devices, and this is what makes it so special. The Bernina Express is the highest-altitude way to cross the Alps by train all the year round without using tunnels.