Zurich, Switzerland (CNN) -- As rail travel grows ever more popular in Switzerland, the country's most important rail station is undergoing a bold transformation to keep up with increasing demand.
The Swiss are among the world's top train travelers, second only to the Japanese. Last year, the average Swiss person traveled about 2,000 miles by rail; most of those journeys happened during the morning or evening rush hour.
Passenger flows have increased by 30% over the last seven years and 350,000 people now use Zurich's central train station every day.
"You can say the Swiss are train addicts," says Daniele Pallecchi, of Swiss Railways.
He adds: "Within the last seven years we had a big increase of passengers using the trains in Switzerland and here in Zurich as well and we need more capacity on the rails and on our trains. So we are building now the cross-city link.
"The cross-city link ensures bigger capacity and shorter time of traveling."
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When finished, Zurich's cross-city railway line will be over six miles long and will form an integral part of the Swiss network's inter-city axis. It is Switzerland's biggest urban project.
At its heart is the new underground "through" station -- or "Löwenstrasse" -- built beneath the main station. Here, two new platforms will serve four tracks that will eventually be connected to the main train station.
Over the past four years men and machines have been digging under two rivers and a fully functioning rail hub, creating a hole the size of two football fields.
"It's the most challenging job you can imagine," says project director Roland Kobel.
"It's not only railways ... almost every section of engineering is realized here: we have tunneling, we have open cuts, we have deep borings."
Kobel says between 500 and 700 people are working on the project at any time, building below ground while the old station continues to function above.
Swiss Railway reps say that while work on the cross-city link continues full bore, 98.5% of passengers still reach their destination on time.
It's estimated that as many as half a million people will use the station each day by 2020. That's why the cross-city link is crucial: once completed, in 2014, it will match that increase by doubling the capacity of the station.