Elstree to St. John's Wood Cable Tunnel

The Elstree to St. John's Wood Cable Tunnel, known as The London Connection during construction, is a 20 km long, 3m wide tunnel beneath northwest London. Constructed between 2001 and 2005, the tunnel carries high voltage transmission lines from Elstree sub-station in Hertfordshire to Lodge Road sub-station in Westminster at a depth of 20m below street level. The tunnel runs beneath the A5 road for the majority of its length, and houses a single 400 kV power transmission circuit with a rated capacity of 3700 A. There is provision for a second circuit to be installed in the future. A remotely operated, battery-powered monorail system runs along the tunnel, allowing the cables to be inspected remotely using infrared cameras.

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