The 35.4-mile will speed train passengers from Milan to Zurich in less than three hours.
After 15 years of construction, the world's longest tunnel is taking shape.
The passageway will form the lynchpin of a new network between northern and southeastern Europe.
The world's longest tunnel will take shape deep beneath the Swiss Alps on Friday when a giant drilling machine clears a path for a key high speed railway through the heart of Europe.
"The Gotthard will forever be a spectacular and grandiose monument with which all tunnels will be compared," said Swiss Transport Minister Moritz Leuenberger.
The 57-kilometer (35.4-mile) Gotthard base tunnel will form the lynchpin of a new network between northern and southeastern Europe that could shift truck freight onto rail and decongest the Alps in central Switzerland when it opens in 2017.
Passengers will ultimately be able to speed from the Italian city of Milan to Zurich in less than three hours and further north into Germany, cutting the journey time by an hour.
But the 9.8 billion Swiss franc (7.0 billion euro) tunnel is also the fruit of a popular wave of concern about pollution in the Alps with booming road traffic transiting from neighboring countries.
After 15 years of construction work, the 9.5-meter (31-foot) wide drilling machine will bore through the remaining 1.5 meters of rock to join two ends of the tunnel some 2,000 meters under a mountain.
The stage-managed event, attended by 200 dignitaries 30 kilometers along the tunnel, will be broadcast live on Swiss television and watched by European Union transport ministers at a meeting in Luxembourg.
But the spotlight will also fall on some 2,500 tunnel workers, many of whom will be feasted at a celebration just above the breakthrough point in the mountain village of Sedrun.
Eight have died since construction of the new tunnel began 15 years ago, blasting and boring through 13 million cubic meters of rock in hot and humid conditions.
The Gotthard tunnel will exceed the 53.8-kilometer Seikan rail tunnel linking the Japanese islands of Honshu and Hokkaido and the world's longest road tunnel, the 24.5-kilometer Laerdal in Norway.
Switzerland struggled to convince skeptical European neighbors to support its ambitious and costly TransAlpine rail plans in the 1990s.plans But they gained added weight in a shock referendum result in 1994 when Swiss voters supported a green motion to stop heavy trucks crossing the Alps -- including the expanding flow of transiting EU goods traffic.
A nationwide opinion poll published on Wednesday suggested that that sentiment is undimmed.
Sixty-seven percent of those surveyed support a ban on truck traffic through the St Gotthard road tunnel, according to the poll commissioned by an Alpine environmental lobby group.
In recent years, Austria, France and Italy have set in motion two similar and monumental rail tunnel projects through the eastern and western Alps.
Around 300 trains should be able to speed through the Gotthard's twin tubes every day, at up to 250 kilometers per hour (155 mph) for passenger trains, according to planners.
The current aging and narrow 15-kilometer tunnel higher up the flanks of the St. Gotthard can cope with just a fraction of that capacity at less than half the speed.
It was nonetheless a global engineering feat as well when it was completed 128 years ago although it claimed the lives of some 200 tunnel workers.