Your challenge, should you choose to accept it, is to create an ATL scenario in which the London Underground is still the largest rapid transit system in the world to the present day, in at least one of these aspects:
a) Total system length,
b) Total number of stations, and
c) Annual ridership.
The POD must be after 1900, but besides that, there are no other stipulations (although nuclear war is distasteful). Are you up for the challenge?