Potters Bar Tunnel

The Potters Bar Tunnel is a railway tunnel on the East Coast Main Line in the United Kingdom between Hadley Wood and Potters Bar railway stations; at 1,214 yards (1, 110 metres) in length, it is the longest on the line. Its two parallel bores carry four tracks beneath high terrain and the M25 motorway. The lines tolerate speeds of 75 miles per hour (121 kilometres per hour) on the slow tracks and 105 mph (169 km/h) and 115 mph (185 km/h) on the fast tracks, with a continual gradient 1 in 200 (5‰), which is uphill for trains heading away from London. The first bore of the tunnel was finished in time for the opening of the line in August 1850. Despite permission having been granted by Parliament in 1882, a second bore was not opened until 1959.

Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).