Mentor to Jena - Louisville & Nashville
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This former Louisville & 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 rejoing 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 to Friendsville.  Friendsville has a small historical monument to the rails, 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 Rd. that parallels US 321 in the area that runs along the former ROW.
 

Stone bridge piers at Louisville, on the banks of the Tennessee River.  Louisville, TN is a settlement and marina, not really a town.  Aerial photo of these piers is shown above.  Topo of this area is shown below.
Thanks to Mike Palmer for the photo and information on this page.
This photo shows the site of the junction at Mentor. The abandoned line curved off to the center right near the brown building in the background.  This view is facing southwest.
Topo of area in pictures at top of page, showing tracks still in place.