Switzerland’s 57-kilometre (35-mile) Gotthard Base Tunnel runs under the Alps from Erstfeld in the central canton of Uri, to Bodio in the southern Ticino canton.
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.
Seventeen years and more than 12 billion Swiss francs ($12 billion, 11 billion euros) later, the Gotthard Base Tunnel is set for its inaugural journey.
German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minister Matteo Renzi, along with Swiss officials, are due to be on board for the ceremonial first run.
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.
The new route also aims to make rail freight more efficient — partly by supporting heavier cargo, which should reduce the number of diesel-guzzling lorries on the roads, improving traffic and curbing pollution.
The number of daily rail passengers is expected to increase from the current rate of 9,000 people to 15,000 by 2020, according to the Swiss federal railway service.
European Union transport commissioner Violeta Bulc last week described the new tunnel as a “godsend for Europe” which will serve as a “a vital link connecting Rotterdam (and) Antwerp with the ports of the Adriatic”.
The Gotthard project was largely made possible by technical advances in tunnel-boring machines, which replaced the costly and dangerous blast-and-drill method.
The primary machine used to make the Gotthard tunnel was roughly 410-metres long and functioned like a mobile factory.
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.
According to the Swiss rail service, it took 43,800 hours of non-stop work by 125 labourers rotating in three shifts to lay the tunnel’s slab track.
When it officially opens, the Gotthard will surpass 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 will be bumped into third place.