Simplon Tunnel
The first tube of the Simplon Tunnel was completed after eight years of construction and, along with the continuation of the line to Domodossola, it was opened in 1906. The connection to Domodossola included the "Varzo elicoidale", which was probably the longest spiral tunnel in the world. The Simplon tunnel was electrified with 3000 Volt 16 Hertz three-phase AC power from the beginning of operations until 1930.