World's longest rail tunnel nears completion

The Gotthard Base Tunnel was first conceived of in 1947. Photo / AP

It's been almost 70 years in the making, but the longest rail tunnel in the world is only days away from opening.

The Gotthard Base Tunnel in Switzerland runs under the Swiss Alps for 57km and is expected to dramatically reduce travel time across Europe.

While this great feat of engineering was first conceived in 1947, construction only began 17 years ago - cutting through 28.2 million tonnes of rock at a depth of about 22.9km.

The north portal of the Gotthard Base Tunnel in Erstfeld, Switzerland. Photo / AP

The project cost about $17 billion, as well as the lives of eight workers.

It will remove a natural barrier to trade and tourism, as the mountain range it passes through is located at the crossroads of four countries.

Workers walk near a support train inside the east section of the Gotthard Base Tunnel. Photo / Bloomberg

It is expected to reduce travel time between Zurich and Milan to two hours and forty minutes - an hour quicker than the trip currently takes by rail.

"They have been boring through really hard rock," former geotechnical engineer Claire Smith told NBC News.

"This isn't like a [subway] line running a few meters below the surface.

"We're talking depths that are measured in kilometres. Working down there is like going down a mine, it gets warmer as you go further in."

Workers stand on a scissor lift as they fix cable support trays to the access tunnel wall inside the Gotthard Base Tunnel. Photo / Bloomberg

German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minister Matteo Renzi will attend the tunnel's opening ceremony on June 1.

Test runs will be carried out over the next few months, before full service starts in December.

It displaces Japan's 5.8km Seikan tunnel as the world's longest rail tunnel, with the 50.5km Channel Tunnel that links England and France dropping into third place.

- nzherald.co.nz