Norfolk Southern Cumberland Gap Tunnel - Trains Magazine

This tunnel is still owned by CSX and was originally bored by L&N or a predecessor, however the trackage on the other side has been abandoned and so CSX still uses the Harlan switchback (last mainline switchback in the US).

SOU recieved trackage rights to reach its branches around Middlesboro, which NS still uses AFAIK. 

As for safety, lots of hype without connection to reality. Different tunnel linings are pretty common as tunnels pass through various types of rock. This tunnel was bored in 1888 and collapsed twice in 1894 and 1896.