The London Underground, also know as the Tube or just the Underground is considered the oldest rapid transit system in the world. It incorporates the world's first underground railway, the Metropolitan Railway, which opened in 1863. The London Underground is also the first line to operate underground electric traction trains.

The system serves 270 stations and has 400 km of route. It is an interesting fact that only 45% of the system is actually "underground" in tunnels.