The Second District is known as the "Rathole" due to steep grades, 27 tunnels, and numerous curves which were once this line's signature. While several projects over the span of 60 years eliminated several problem areas, the Southern Railway's line improvement project between 1959 and 1963 is the best known. This project saw numerous cuts and line relocations to bypass tunnels and reduce the steep grades and tight curves. Only Tunnels #22 and #24 at Nemo, Tennessee and Tunnels #25 and #26 at Oakdale remain on the line; all but #25 were built new in the 1960s.