This is probably the most beautiful railway tunnel in the world, and it's in Australia. It's also disused, which is a crying shame. The Helensburgh Tunnel opened in 1880, but quickly became the victim of advances in technology.
When the steam locomotive became popular, two lines were required instead of just one. The tunnel only had room for one line, and so it was cut from the route. Until recently, tourists were allowed to come and walk through it, but vandalism put paid to that practice.