Officials from Germany, France, Italy and Switzerland will attend the event

When Swiss engineer Carl Eduard Gruner came up with the idea to build the world's longest rail tunnel under the Alps in 1947, he predicted the visionary project could be finished by the start of the 21st century.

He would turn out to be off by 16 years.

Gruner's sketch and technical paper called "Journey through the Gotthard Base Tunnel (GBT) in the year 2000" planted the seeds for a venture that will be launched this week and aims to transform travel through the heart of Europe.

Constructing the 57-kilometer rail tunnel took just over 12 billion Swiss francs ($12 billion) and some 2,400 workers, according to Swiss government statistics.

More than 28 million metric tons of rock had to be excavated from the mountain near the Gotthard pass.

The bumpy road from Gruner's initial idea to the official start of construction in 1999 included bureaucratic delays and concern over the project's financial viability.

Switzerland's federal government set up a committee in 1963 to look into the building of an alpine base tunnel but opinions were split over various proposals, before an official body eventually ruled in 1983 that the project was "not urgent."

Momentum in favor of the GBT built again ahead of a 1992 referendum, which supported the project, but public outcry about the expense put work on hold.

Eventually, a November 1998 referendum closed the deal, when 64 percent of Swiss voters approved the final plans along with funding schemes, including a new road tax.

Will overtake Japan

"It's a big engineering achievement," said Martin Knights of the London-headquartered Institution of Civil Engineers.

The tunnel runs from Erstfeld in the central Swiss canton of Uri, to Bodio in southern Ticino canton, and 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, including by supporting heavier cargo, which should reduce the number of diesel-guzzling lorries on the roads, in turn improving traffic and curbing pollution.

The number of daily rail passengers is also expected to increase from the current rate of 9,000 people to 15,000 by 2020, according to the Swiss federal railway service.

In a study of the GBT's possible economic benefits, Swiss bank Credit Suisse forecast a range of positive impacts driven by an easier movement of goods, growth in day-trip tourism and higher property values in the affected cantons linked to increased accessibility.

German Chancellor Angela Merkel, French President Francois Hollande and Italian Prime Minister Matteo Renzi, along with Swiss officials, are due to attend Wednesday's grand opening, which will feature a ceremonial run.

When it opens, the GBT will overtake Japan's 53.9-kilometer Seikan tunnel as the world's longest train tunnel and bump the 50.5-kilometer Channel Tunnel that links England and France into third place.

By the numbers

The world's longest rail tunnel, which runs under the Alps and took 17 years to build, will be inaugurated on Wednesday. Here are four key numbers about the project, called the Gotthard Base Tunnel (GBT).

57: At a length of 57 kilometers the GBT will overtake Japan's 53.9-kilometer Seikan tunnel as the longest rail tunnel in the world.

1882: That was the year the first rail route through the Gotthard pass opened, a landmark in Swiss history that helped boost trade through the Alpine nation, which had previously been arduous to cross.

43,800: According to the Swiss federal rail service, it took that number of hours of nonstop work by 125 laborers rotating in three shifts to lay the GBT's slab track.

2 hours and 40 minutes: That is the estimated length of time it will take to get from Zurich to Milan once the GBT opens full service in December, roughly an hour shorter than the journey currently takes.