Rail bosses say safety comes first in the soon-to-be-opened Gotthard Base Tunnel, the longest in the world at 57 km. (SRF/swissinfo.chexternal link)
The tunnel is at the heart of the New Trans-Alpine Railway Link (NEAT) project, geared towards improving the north-south rail link through the Alps.
Base tunnels enable flatter and shorter rail routes so trains can travel at higher speeds and pull more weight. The link is only 550 metres above sea level at its highest point - much lower than existing routes through the Alps.
For ten years, hundreds of workers have braved tropical temperatures underground to get the tunnel finished. It has a system of two one-way tubes, linked by emergency escape routes every 325 metres. To make it watertight, the tunnel vault was covered with a waterproof mantle, allowing water to trickle down inside the walls and be channelled out through pipes under the tracks.