We would hazard there's not a single person reading this post who hasn't had their rush-hour standing snooze interrupted by an irate tube driver imploring his passengers not to lean on the doors. Looking at the faces of those nearby as the train struggles to get going, it's clear some think the driver is overreacting a tad as he or she berates a wedged-in punter for what seems a trivial offence.
High time to find out exactly what catastrophe befalls such a train bulging with commuters. We contacted Ben Pennington, head of the press desk at London Underground, and he duly extracted the required knowledge from the 'rolling stock professional head' (which we like to picture as a giant brain buried deep beneath Paddington station).
All tube train doors have a safety circuit, known colloquially as an interlock but more fancily as a 'sensitive edge'. If the doors are open to any degree, the circuit is broken and the power to the train's propulsion system is cut. Ostensibly this is to prevent someone being dragged along half in and half out of a train, and all the mayhem that would evoke.
However, the design of the train doors is such that someone leaning heavily on the door is applying a pressure to the connecting points, which the train’s system interprets as a gap in the interface between the door and the frame. In short, the doors don’t know if the circuit is broken by someone or something trapped between them or someone leaning on the inside.
But are some lines more susceptible than others? The brain says no: from a passenger’s perspective it’s more noticeable on the older D and C stock of the District, Hammersmith and City and Circle lines, as those are able to move off a short way and then come to a halt if the door senses an obstruction. This is why a train will sometimes start to leave a station then shudder to a halt, causing the ire of the driver to magnify exponentially with each vibration.
On newer stock, such as the 09TS on the Victoria line, the door circuitry operates on the same principle but the train won’t even begin to move off if it detects a break in its sensitive edge.
So there you have it: the next time you hear a driver demanding you shift your backside from its comfy berth wedged between the tube doors and four other cheeks, he really does mean your ass is going nowhere if you don't move it.