The Apennine Base Tunnel is a railway base tunnel 18.507 km (11.5 miles) long on the Bologna-Florence Direttissima line in central Italy. The tunnel was opened on 21 April 1934 shortening the rail connection by 35 km compared to the old railway line over the mountain range. In the middle of the tunnel there is a switch-over and a 450 m long non-public station accessible by an adit.
Train 904 bombing
On 23 December 1984 17 people were killed and 250 wounded by a bomb on the Naples-Milan Express, Train 904, which exploded in the tunnel. The attack is attributed to Cosa Nostra (the Sicilian Mafia).
The Italicus Express bombing in 1974 was on the same line, though not in the tunnel.
References
Apennine Base Tunnel Wikipedia(Text) CC BY-SA