This former Louisville and Nashville line appears to have been either an alternate route or earlier route to the still-active CSX (L&N) mainline from Knoxville to Cartersville, GA via Etowah, TN. This line diverted from the CSX main at Mentor, ran through Louisville, Airline, Marmor, Friendsville, Rega, Dora, and Greenback before rejoining the main at Jena.
It is not known when this line was abandoned, but it apparently was in the mid-1980s.
The line can be followed relatively easily between Mentor and Friendsville. Friendsville has a small historical monument about the railroad, and there are some more bridge abutments in the area. It is difficult to follow the line southwest from there. There is a poorly maintained road called Kiser Station Road that parallels US Route 321 in the area that runs along the former ROW.
Thanks to Mike Palmer for contributing information about this route.