Starting in 2015, certain lines on the London Underground will operate all through the night, joining cities like New York and Berlin that already have 24-hour service. Even though the all-night tubes will only run on Fridays and Saturdays while largely serving central, west, and north London, the Night Tube service is still a welcome improvement for locals and visitors who enjoy staying out after midnight, currently the time when most underground trains stop running.
Transport for London released a map of the Night Tube service, which will go into effect on Sept. 12, 2015, just in time for the Rugby World Cup in England. There will be six trains each hour on the entire Jubilee (gray), Victoria (light blue), and most of the Piccadilly (dark blue), Central (red) and Northern (black) lines.
See here:
TFL
London already has night buses that run after hours, but this mode of transport tends to be less efficient than the rails because there are so many stops on each bus route, and placed fairly close together.
According to managing director of London Underground Mike Brown, half a million people in London currently use the Tube after 10 p.m. on the weekend. The 24-hour train service will almost definitley be a boon for London's late-night scene.
For reference, here's the what the regular day service Tube map looks like. There are clearly large splotches of London that won't have Night Tube service.
Transport for London