When the twin tubes of the Gotthard Base Tunnel (GBT) running north-south under the Alps are officially inaugurated in a ceremony on June 1st, Europe will mark the completion of the world's longest and deepest train tunnel.

"The entire [Swiss] Federal Council, also the heads of state and government of our neighbor countries, as well as the transport ministers of the countries along the Rotterdam-Genoa freight corridor," are expected at the June 1 opening ceremony, according to a press release from Alptransit, the subsidiary of the Swiss federal railway company that led construction of GBT's rail infrastructure, and "a total of around 600 artists will perform."

GBT has been a long time coming. Swiss engineers first began thinking about building train tunnels under the Alps in 1947. In 1963, a commission started considering options. Swiss voters green-lighted the project in a 1992 referendum, and engineering work began on-site in 1995. Tunnel-boring operations started in 2002 simultaneously from the route's north and south ends, and were completed in March 2011, "after removing more than 28 million tons of rock from deep under the Gotthard mountains with tunnel-boring machines and explosives," according to Alptransit chief engineer Renzo Simoni.

Since then, Simoni's engineers have been busy kitting out the tunnels with railway tracks, ventilation equipment and other gear. GBT consists of two big train tunnels spaced about 40 m apart from each other, enabling trains to flow in both directions simultaneously, plus four smaller interconnector tunnels between the main tubes, to enable emergency evacuations in case a fire ever breaks out in one of the main tubes.

The train tracks running through GBT were ready for their first operational tests in October 2015. Extensive additional test runs of freight and passenger trains will be conducted over the remainder of 2016. Commercial train service is set to begin in mid-December 2016 - a full year earlier than had been anticipated at the project's outset.

Heavy engineering

The 57-km- (35-mile-) long north-south tunnel route, flat and horizontal, runs from Erstfeld in Canton Uri to Bodio in Canton Ticino. At some points, the vertical distance from the tunnel to the mountain peaks above is 2.3 km, comparable to some of the deepest mines on Earth. The temperature at those depths reaches 46 degrees C in the absence of ventilation.

The tunnels were made using enormous tunnel-boring machines built by Germany's Herrenknecht AG, the world's leading supplier of such equipment. The machines had to bore through many very different kinds of rock, ranging from dangerously soft to very hard, and the tunnels had to be engineered to stay stable as the boring-machines progressed, despite the crushing weight of millions of tons of rock bearing down from above - not a trivial challenge.

While the tunnels were neither easy nor cheap to build - Switzerland paid a total of 12.2 billion Swiss francs ($12.2 billion, 11.0 billion euros) to get the job done - the cost overrun compared to the initial estimate was only 21 percent in inflation-adjusted terms.

"The Gotthard Base Tunnel is close to median performance as regards cost overrun, but given that the tunnel is longer than most tunnels and has taken longer to build, I would say that 21 percent is not a bad number," said Bent Flyvbjerg, a researcher at the University of Oxford's Saïd Business School. Flyvbjerg told DW that transportation megaprojects in Europe typically overshoot initial budget estimates by about 27 percent.

Project manager Simoni has said that the costs of a unique, multi-decadal megaproject like GBT cannot be calculated definitively in advance, since technology changes over time, and conditions on the ground (or, in this case, in the ground, under the mountains) cannot be fully known in advance. Given the challenges, Jobst Fiedler, a researcher at Berlin's Hertie School of Governance, sees GBT as a case study of sound project management and good financial governance.

Still, 12.2 billion francs is a lot of money, and Switzerland didn't set out to build the word's longest and deepest train tunnels just to get into the Guinness Book of Records. What made the Swiss decide to build GBT, or "Gottardo," as it's informally known?

Freight trains rollin'

Environmental concerns were the main drivers of the Swiss government's decision to build Gottardo. The project is meant to help shift freight from trucks onto trains, and reduce the environmental footprint of transalpine freight transport by rerouting it underground.

Instead of schlepping millions of tons of freight up the steep Alpine mountainsides and over the high mountain passes on the way from northern to southern Europe by diesel engine powered truck, or via the existing alpine train route, some 200 to 250 electrically powered freight trains will speed quietly along a horizontal track deep under the mountains each day, transporting around 40 million tons of freight each year.

Passenger trains will also use the new tunnels, reducing travel time between Zurich and Lugano by about 45 minutes.

Gottardo is also expected to have economic benefits, mostly because the new route will enable more freight and more passengers to be transported more quickly and cheaply, compared with the high-mountain route.

The full benefits of the project, however, will have to wait until Gottardo's sister project, the 15.4 km long twin-tube Ceneri Base Tunnel (CBT) under the southern Alpine mountain ranges, is completed in 2019. CBT will be an important feeder route for Gottardo, since the existing steep-grade train route over Monte Ceneri isn't suitable for high-speed freight trains.

Even before completion, Gottardo has benefited the Swiss economy - not just because of the jobs created in construction, but also because of "tunnel tourism." Since GBT construction began in 1996, a total of more than a million tourists have come to gawk at the mammoth work-in-progress, according to Alptransit.

In future, with Gottardo and CBT in service, tourism officials expect increases in the tourist traffic between German-speaking northern Switzerland and Italian-speaking southern Switzerland, especially in the region of Tessin.