CHIOMONTE, Italy — To get to the site of an exploratory tunnel now being bored into Italy’s northwestern Alps, visitors must first pass through a police checkpoint, then a military one. Tall fences topped with barbed wire roll along the mountainside. Armored vehicles cross paths with jeeps on winding roads lined by vineyards.

In an area known for picturesque villages, winter skiing and summer Alpine excursions, the fortified construction yard is a jarring juxtaposition, betraying the bitterness of a two-decade battle over plans to build a high-speed train link between Italy and France.

Over the years, the saga of the train line has been punctuated by episodes of popular resistance and colorful, thousands-strong demonstrations, but also by violent clashes, nighttime acts of sabotage and even accusations of terrorism.

Local people have long resisted the rail link, fearing damage to aquifers as well as the possible release of asbestos and radioactive materials during the excavation. They also questioned the economic sense of a project that required an initial outlay of nearly $12 billion.