The mile-long tunnel, which has been in place for more than 175 years, has kept Lancashire and Manchester connected.
Sough Tunnel, approximately 1,842 metres long, has two shaft turrets that protrude out of the Darwen moors.
Sough Tunnel(Image: Facebook)
The turrets were originally used as an outlet for steam trains, but now help to release air pressure created inside by moving trains, maintaining air quality for passengers and preventing damage to the walls inside.
In 1984 the entrance to Sough Tunnel was Grade II listed and in 2020 the tunnel was repaired using a novel method due to engineers being unable to access the interior.