With the introduction by 1990, of automatic ticket barriers into all underground stations, including the outlying ones, the tube forms a closed system. The idea is that it is not possible to enter the tube system without a valid ticket, and not possible to leave it without a ticket covering the journey.
What happens if you miss your stop?
Unlike the Paris Metro and the New York Subway, it is always possible on the tube, to retrace your steps by boarding a train in the opposite direction. To do this, you need to stay inside the ticket barriers and find the other platform. Many stations have platforms which lie between the two track directions, and it is merely a matter of crossing over the central divide to find the trains going the other way. In other cases, you may need to ascend or descend a stair case.
The tube map shows which stations serve more than one line as interchanges. Again, to use an interchange does not require passing out and in ticket barriers, with a few exceptions:
- King's Cross St. Pancras Subsurface platforms (Hammersmith and City Line, Circle Line, Metropolitan Line) is separate from the underground station for the deep tube lines (Northern Line, Piccadilly Line, Victoria Line).
- Hammersmith station is actually two stations either side of a road with pedestrian traffic lights, but is shown as an interchange.
Note, that there are instances of separate stations with the same name, which are shown as separate stations (again no interchange, you have to exit the ticket barriers to change lines).
For those with disabilities
Some stations have cross platform interchanges.