In Vault Of Death, the London Underground is a disused transportation system. It was closed after the introduction of overhead monorail lines.
Vault of DeathEdit
In a last ditch attempt to rescue Lambert from the Bank of England's vault, Grandma Tracy came up with the idea that the International Rescue team might be able to reach the Bank via the London Underground System, even though it had been closed down for many years. In the end, Alan and Virgil Tracy had to travel underground through five stations of the derelict subway system in order to reach a point directly underneath the Bank.