Which Alpine rail tunnel between Switzerland and Italy opened in 1906?

The Alpine rail tunnel between Switzerland and Italy that opened in 1906 was the Simplon Tunnel.

It links Brig in Switzerland with Iselle di Trasquera, near Domodossola in Italy, beneath the Simplon massif. Construction of its first tube began in 1898, and the two sides met in February 1905. The tunnel officially opened on 19 May 1906, creating a major north–south route through the Alps.

The original tunnel was about 19.8 kilometres long and was built as a single-track tube, with a parallel service passage. That passage was later expanded into the second running tube, which opened in stages around 1921–22. The tunnel was exceptionally low for an Alpine crossing and used electric traction from its early operating period.

A common mix-up is with the Lötschberg Tunnel, another important Swiss Alpine route, or the Mont Cenis Tunnel between France and Italy. The Simplon Tunnel is also associated with the later Simplon-Orient-Express route.