David, You are correct. This was the Lost Gap Tunnel through Tunnel Hill. The Second Brigade, Third Division, XVII Corps, Col. Benjamin F. Potts commanding, was assigned the task of destroying the railroad from Meridian to Chunky. They went through the long tunnel and were only able to do slight damage as it was arched over with brick and reported "the masonry showed that an expert had had a hand in laying up the brick."
The tunnel lasted until November, 1963 when the railroad "daylighted" the tunnel - dug it out and left a deep cut through the hill.