Travelling Underground > A century of Underground trains
Underground trains are designed to carry large numbers of people as rapidly as possible over fairly short distances. Passengers need to get on and off quickly and easily, but also want to travel in reasonable comfort. There has to be a balance between sitting and standing space inside the train.
The Metropolitan, District, Circle and East London lines run in tunnels with enough clearance to accommodate overground size trains. The circular tunnels on the seven deep-level Tube lines are smaller: 3.6m (12 feet) in diameter.