At a neat 600 yards in length, Neidpath/South Park/Peebles/Castle Hill Tunnel - take your pick! - threaded the Symington Biggar & Broughton Railway through a projecting shoulder of rock under South Park Woods.
Originally proposed by the Caledonian Railway in 1846, the scheme met with fierce opposition from the North British Railway, operators of the line to the east of Peebles. It eventually saw steam in 1864.
In the early part of the 20th century, Nobel Prize winning scientist C T R Wilson used the tunnel - after traffic had ceased for the night - to conduct experiments with his cloud chamber to prove (or not) the existence of cosmic radiation.
The line saw a considerable amount of traffic during World War II and it is often rumoured that the tunnel served as a hiding place for the Royal Train while the King and Queen were inspecting the damage caused by the Blitz on Clydeside. However the fact that the route is single track and a stabled train would effectively block it casts much doubt on this story.
The tunnel remained operational until 7th June 1954 although the passenger service had ended four years earlier. It was latterly used by the Wemyss & March estate for the movement of timber.
Costing £14,431 to drive from either end and a central shaft, the bore is brick-lined for most of its length but with masonry portals. Although the western section is straight, a southerly curve is encountered around 150 yards from the east end. There are no refuges but, close to the centre, an unlined chamber is cut into the northern wall.
Today Neidpath Tunnel benefits from a Grade B listing.