We first visited the railway tunnel in 2002, and again in 2008 when we took pictures of it. It is quite an interesting tunnel since it has defensive positions for the fortress placed on top of the hill.
The railway was planned in 1853, building started in 1857. Due to the demanding landscape many bridges and tunnels are needed, so in 1863 the railway from Charleville (France) to Namur (Belgium) is opened. The tunnel beneath the fortress of Charlemont was build in 1862. The tunnel gave the attackers of the fortress an advantage, so it had to be defended. It is the only defended railway tunnel I know. It is defended by shooting positions, bunkers and pillboxes. Maybe there is a connection to the fortress, but we did not find any. The fortress was no longer used as defensive position after 1900, but is still used by the military.
The tunnel is used by regional freight and passenger traffic. It is not used much, but there are plans to reopen the line in Belgium, making it an international transport axe. This should bring new life into the area.