This is the Hagan tunnel southern portal about 10 miles west of Pennington Gap at 36.706559, -83.288597  in Lee County VA.  The tunnel, over 6,000′ long, runs from Hagan through Cumberland Mountain to Chevrolet KY, originally as part of L&N’s Cumberland Division.  The tunnel was constructed in the late 1920s.  There is also a single reference to another tunnel here at Hagan breaching the mountain higher up in a private or community effort back around the turn of the 20th century.  It was used by wagons for commerce between the states (we couldn’t find it).
This place is also interesting in that there are the two wyes (I was corrected on this…it’s a switchback) near this portal.  The CSX train comes over the top of the tunnel entrance going in a westerly direction, moves off to the first switchback, then backs down to the other one, which allows it to head into the tunnel going more or less northward.  All very complicated and a headache, I’ve read, for CSX, but since this is a lightly used, low speed line, they feel the cost of clearing all this up would not be profitable.  It takes about an hour for the train we observed to make the complete transition.  This was (I’m guessing) a 90-car unit.  At that length, it was completely in the tunnel before it came out the other side (math – coal car: about 50′ or so and the two engines about 75′ each…making the train about .8 mile in length).