London Underground

The London Underground, also known as just the Underground and nicknamed the Tube, is London's subway system, one of the oldest in the world.

Principles
The London Underground serves as a public rapid transit system for Greater London. It has been said that the subway structure would circumnavigate the globe two and a half times if it was all laid out end to end, though the veracity of thia claim isn't clear.

Many of the tunnels and stations have a distinctive curved shape, which gave rise to the Underground's nickname, the Tube.

As it has been modified often and spreads out over the city like a spider web, there are apparently a lot of abandoned lines and stations which not even the maintenance workers know out. Some of these abandoned sections have been adapted by the Anglicans for their own purpose, made into vaults and training grounds, though the underground labyrinth formerly used to test new recruits for Necessarius was decommissioned as it was too dangerous and too many people kept dying there.

Background
London was the first city in the world to dig subway tunnels.

At some points in the past, the Anglicans adapted some of the abandoned sections of the Underground for their own purposes, though their training grounds for new recruits were later decommissioned due to being too dangerous and too many people dying there.

British Royal Family Arc
When New Light were initiating their operation in London, Bayloupe was positioned near an entrance to one of the Underground's stations. On being confronted by the Amakusas following the disturbance caused by Lessar, Bayloupe fled into the Underground, pursued by Itsuwa.

After a brief fight in the station, having knocked Itsuwa down with an explosion from dust and air gathered and launched with her Steel Gloves, Bayloupe escaped through the tunnel, but ended up being redirected into the abandoned training grounds, where she eventually succumbed while protecting her case.

Coronzon Arc
During the Crowley's Hazard invasion of the United Kingdom, the London Underground was the site of the second stage of Kamijou Touma and Aleister Crowley's battle against Isis-Demeter, fused with Orsola Aquinas. Touma had Aleister lure Orsola into the Underground in order to limit her flight capabilities, but Isis-Demeter soon spread its plants throughout the tunnels, giving it a different advantage.

Trivia

 * In the anime, a different roundel logo is used from that of the real-life London Underground, likely for copyright reasons.
 * Toaru Majutsu no Index III E11 16m 02s.png the anime, when Bayloupe tries to escape the Amakusas and is pursued by Itsuwa, Elizard's name is plastered in one of the tunnels, indicating that her name is being used as one of the Underground's lines or the station.  The blue color scheme parallels the real-life, which also includes Victoria station as a stop. The (also known as Crossrail), meanwhile, is a real-life example of a London Underground structure named after a living monarch.