- The 57-km long Gotthard Base Tunnel was conceived in 1947
- The tunnel took 17 years to complete and cost 11 billion euros
- The tunnel will cut the train journey from Zurich to Milan by an hour
With political unity on the continent shaken by a massive influx of migrants and the looming threat of Britain's EU departure, Swiss president Johan Schneider-Amman said the tunnel would "join the people and the economies" of Europe.
He spoke before the 57-kilometre (35-mile) Gotthard Base Tunnel (GBT) made its ceremonial first run with European leaders on board.
The passengers included German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minister Matteo Renzi.
It runs from Erstfeld in the central canton of Uri, to Bodio in the southern Ticino canton.
Travel through the Alpine region, by rail or by road, requires taking a zigzag and undulating route.
The Gotthard Base Tunnel was designed to offer a better option for both private travellers and commercial freights.
When the full service opens in December, the tunnel will shave the train journey from Zurich to Milan in northern Italy down to two hours and 40 minutes, roughly an hour less than it currently takes.
It should also make rail freight more efficient -- partly by supporting heavier cargo, which should reduce the number of smoke-spewing lorries on the roads, in turn improving traffic and curbing pollution.
Light After Decades Wait
The rough design for a rail tunnel under the Gotthard Pass was first sketched by Swiss engineer Carl Eduard Gruner in 1947.
But bureaucratic delays, concerns over the cost and other hurdles pushed back the start of construction until 1999.
The work took 17 years at a cost of more than 12 billion Swiss francs ($12 billion, 11 billion euros).
According to the Swiss rail service, it also took 43,800 hours of non-stop work by 125 labourers rotating in three shifts to lay the tunnel's slab track.
The ambitious venture was largely made possible by technical advances in tunnel-boring machines, which replaced the costly and dangerous blast-and-drill method.
It cuts through rock and throws the debris backwards while simultaneously placing the pre-formed segments of concrete that form the shape of the tunnel.
A separate system grouts the pieces together.
With its official opening, the GBT has surpassed Japan's 53.9-kilometre Seikan tunnel as the world's longest train tunnel.
The 50.5-kilometre Channel Tunnel that links England and France has been bumped into third place.