Potters Bar Tunnel
Railway tunnel in Hertfordshire
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 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 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.