In 1884 a they started building a tunnel through a mountain, they dug from both ends, length of the tunnel was 3443 yards, that's just 77 yards short of 2 miles, and they were only 1 1/2 inches adrift, this was before GPS and lasers, a marvelous feat of engineering.
In 1968 it was closed due to safety issues, it never re opened because there were massive cuts to the rail network, and cost was too much to repair.
Take a look at this maphttps://www.google.co.uk/maps/dir/51.656114,-3.5966475/51.6764683,-email@example.com,-3.5231993,12z/data=!4m2!4m1!3e0
To get from one end of that blue line to the other is 11 miles up the mountain and along winding roads, and would take 28 minutes by car (use streetview in the middle to see the terrain).
And now there are plans to re open the tunnel as a cycle and walking path, subject to safety inspections and what work is needed.
A few people have been in the tunnel before this plan (urbex) and it looks great with the original brickwork.
Why am i interested ? I used to live in Blaengwynfi, and now only live a couple of miles from there.
Great website with videos http://www.rhonddatunnelsociety.co.uk/video