London Underground

The London Underground, 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.

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.

British Royal Family Arc
On being confronted by the Amakusas, Bayloupe fled into the Underground and fought with Itsuwa in one of the stations. She then escaped through the tunnel, but ended up being redirected into the abandoned training grounds.

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.

Trivia

 * In the anime, a different roundel logo is used from that of the real-life London Undergroun, likely for copyright reasons.
 * In the anime, when Bayloupe tries to escape the Amakusas, 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.