A giant drilling machine will complete the world's longest tunnel beneath the Swiss Alps on Friday, paving the way for continuous high-speed rail travel between northern and southeastern Europe.

The ceremonial breakthrough in the 57-kilometre long Gotthard base tunnel through the foot of the Alps is due to take place 30 kilometres from one end and 2,000 metres below a mountain.

Eight of some 2,500 tunnel workers have died since construction of the new railway link began in central Switzerland 15 years ago, blasting and boring through 13 million cubic metres of rock in hot and humid conditions.

By the time it opens for service in 2017, it will exceed the 53.8-kilometre Seikan rail tunnel linking the Japanese islands of Honshu and Hokkaido and the world's longest road tunnel, the 24.5-kilometre Laerdal in Norway.

Although the near 10 billion Swiss franc ($A9.9 billion) project is Swiss, it has fast taken on a continental dimension with the aim of unclogging one of the main north-south commercial routes between Germany and Italy.