We've all seen the hapless Tube newbies ("Tubies", as they shall henceforth be called) hammering away at the 'Open' button on a train, blissfully unaware that the doors will open when the driver bloody well wants them to, and no sooner.
But why do so many Tube lines have these redundant buttons?
Ride on the Northern, Jubilee or Central line, and you'll see the buttons beside every door. On some lines, they even light up when the train pulls into a station, inviting the busy passenger to press away in futile rage. But on every Underground train, the doors are operated by the driver. Yet the existence of the buttons suggests that, at some point, passengers were expected to open the doors themselves. Why aren't we trusted to do so any more?
The reason is speed of entry and exit. In the 1990s Tube bosses realised that dwell time at stations would be reduced if the doors were opened by the driver, rather than waiting for passengers to press the button. The problem was more acute on the Central line, which is unique on the Underground in having both an 'open' and 'close' button. There were stories of malicious ne'er-do-wells closing the doors on people, and an incident at Notting Hill Gate in which a young boy was injured in a door closed by another passenger. These issues, plus the desire to have a standardised system across the entire network, led to the decline of the passenger-operated doors in the late 1990s.
The D78 stock District line trains used to have buttons, and when the trains were introduced the idea was for the doors to be manually operated by passengers at every station. But the lack of ventilation meant that in summer trains were getting unreasonably hot, so in the warmer months the driver would operate the doors. This arrangement lasted until the late 1990s, when the seasonal switch back to passenger operation simply never happened. The buttons inside the carriages were removed during a mid-2000s refurb (the buttons on the outside of the train are covered up by a panel).
Nowadays, while the DLR and Overground trains have buttons, few Underground lines do. But things are changing: the S-series stock, currently serving the Metropolitan and Hammersmith & City but scheduled to cover the District and Circle lines too by 2015, are all fitted with door buttons. While the driver normally controls the doors, when the trains sit for extended periods at a station the practice is to close the door but allow passengers to open it by pressing the button, thus keeping the cool / warm air inside the train.
For an in-depth look at the history of buttons on the Underground, read this exhaustive document (PDF).
So that's the real story behind the buttons. For the rest of us, though, they'll continue to be a handy way of identifying tourists and newcomers to the city.