London Underground

The Metro rail system in London is largest in the world when the overground network is also considered. 

The standard tube network alone consists is of 402 km of railroad. This makes it the 3rd largest in the world. 

There is also a large overground network throughout London. 

There are many instances when a street will have two train stations on it with a 5 to 10 minutes walk between them; Places like frogmore and Eastcoat. 

Plus there are trams and lite robotic trains(DLR). London is so crammed with trains that there is always a station nearby.