Neidpath railway tunnel is a nearly half-mile-long abandoned train tunnel situated close to the town of Peebles and was often known as the Peebles Tunnel. William Scott was contracted to construct the tunnel as allotment of the Peebles-Symington extension. The tunnel was once considered the most notable feature of the rail line and was built under South Park Wood to emerge within an earshot of Peebles. The all-consuming and overwhelming darkness of the tunnel can be eerie. Along certain sections inside there is no natural light and you can see neither the entrance or exit. The tunnel was also used during World War II to transport raw materials for the war effort. It's believed that it was also the hiding spot for the royal train when the king and queen surveyed damages done in Clydebank following a Luftwaffe bombardment, although this has been disputed. The town was known for being a key ship manufacturer during the war.
Source: atlasobscura.com