The Gotthard Base Tunnel, with a length of 57.09 kilometres (35.47 mi) and a total of 151.84 km (94.3 mi) of tunnels, shafts and passages, is the longest railway tunnel in the world,[note 2] with a geodetic distance of 55.782 kilometres (34.661 mi) between the two portals. It is also the first flat route through the Alps or any other major mountain range, with a maximum height of 549 metres (1,801 ft) above sea level, corresponding to that of Berne. It is the deepest railway tunnel in the world, with a maximum depth of 2,450 metres (8,040 ft), comparable to that of the deepest mines on Earth. Without ventilation, the temperature inside the mountain reaches 46 °C (115 °F).
Like the two other tunnels passing below the Gotthard, the Gotthard Base Tunnel connects two Alpine valleys across the Saint-Gotthard Massif: the Urner Reusstal in the canton of Uri, in which flows the river Reuss, and the Valle Leventina, the largest valley in the canton of Ticino, in which the river Ticino flows. Unlike most other tunnels, the Gotthard Base Tunnel passes under several distinct mountain massifs, two of them being major subranges of the Alps, the Glarus Alps and the Saint-Gotthard Massif, with the valley of the Anterior Rhine, the Surselva in the canton of Graubünden, between them. The tunnel passes under these two ranges more than two kilometres (6,600 ft) below the Chrüzlistock (2,709 m (8,888 ft)) and the Piz Vatgira (2,983 m (9,787 ft), near the Lukmanier Pass). While the cantons of Uri and Ticino are part of the German- and Italian-speaking areas of Switzerland respectively, the Surselva is mainly Romansh-speaking.
The north and south portals on the same spring day. Note the prevalence of coniferous trees and snow at the north portal and the absence of them at the south portal.
The Alps strongly influence the European climate – and that of Switzerland in particular – and there can be substantially different weather conditions at each end of the GBT, described by the Ticinese architect Mario Botta: "The light changes at the Gotthard: that of the Mediterranean Sea is not the same as that of the continent, that of the central lands, that of Europe far away from the sea." On average, the temperature is 2 to 3 °C (4–5 °F) higher on the south side than the north side, but on some days, temperature differences are well over 10 °C (18 °F).[note 3]
The north portal lies in the north of the municipality of Erstfeld at an elevation of 460 metres (1,510 ft), east of the Reuss. There, the tunnel penetrates the western slopes of the Bälmeten and Chli Windgällen (although only marginally) before passing below the valley of the Chärstelenbach, a creek in the Maderanertal. From there, the tunnel runs parallel to the small valley of Etzli, below the Witenalpstock. The main crest of the Glarus Alps, which is the watershed between the Reuss and the Anterior Rhine, is crossed below the Chrüzlistock, the crest having an elevation of about 2,700 metres (8,900 ft) at this point. From the crest and border, the tunnel runs parallel to the small valley of the river Strem (Val Strem) before passing below Sedrun and the Anterior Rhine. From the bottom of the valley, the tunnel proceeds towards the valley of the Rein da Nalps (Val Nalps) and passes east of Lai da Nalps, before crossing the Gannaretsch range below the western summit of Piz Vatgira (2,981 metres (9,780 ft)). This is the deepest point of the tunnel, with a rock layer of 2,450 metres (8,040 ft) above it. The tunnel then passes below the valley of the Rein da Medel (Val Medel) and west of Lai da Sontga Maria. After a few kilometres the tunnel crosses the watershed between the Anterior Rhine and the Ticino, just north of Pizzo dell'Uomo (2,525 metres (8,284 ft)). This point corresponds to the main chain of the Alps, and is the main drainage divide between the Rhine and the Po. For a few kilometres, the tunnel passes below two western tributaries of the Brenno in the Valle Santa Maria before crossing the last range, west of the Passo Predèlp (about 2,500 metres (8,200 ft)) and east of Faido. It then follows the eastern slopes of the large Valle Leventina, the valley of the Ticino, for about 18 kilometres (11 mi) to the south portal at Bodio, at an elevation of 312 metres (1,024 ft), just 3 kilometres (1.9 mi) before Biasca, where the Brenno converges with the Ticino.[note 4]
The closest railway stations to the portals are Altdorf and Biasca. The first regularly served railway stations on the base line (as of 2016/17) are those of Arth-Goldau (Schwyz), a railway node with links to Lucerne and Zürich, and Bellinzona (the "Gate of Ticino"), with links to Locarno, Luino and Lugano (via the Monte Ceneri Rail Tunnel). The journey from Arth-Goldau to Bellinzona takes not more than an hour. The station of Altdorf is planned to be served by 2021. There also have been talks of using that of Biasca. The travel between Altdorf and Biasca would last less than 25 minutes.
Accesses to the GBT complex
Erstfeld, north portal, 460 metres (1,510 ft) a.s.l.
Amsteg portal (maintenance access), 507 metres (1,663 ft) a.s.l.
Sedrun portal (maintenance access, bridge over the Anterior Rhine), 1,334 metres (4,377 ft) a.s.l.
Faido portal (maintenance access), 757 metres (2,484 ft) a.s.l.
Biasca, south portal, 312 metres (1,024 ft) a.s.l.