Why You Really Should Stand Clear Of The Doors

Chris Lockie
By Chris Lockie Last edited 56 months ago
Why You Really Should Stand Clear Of The Doors

Packed tube

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.

See also:

Why are there buttons on Tube train doors?

Other Tube mysteries

Photo by t-a-i via the Londonist Flickrpool

Last Updated 28 June 2013

Mark Walley

True story, to test a new sensitive edge for (I think) the new Metropolitan Line trains, they equipped some of the old Met stock doors with sensitive edges then rigged cameras up that would record what happened if people tripped the edge, to see how sensitive they were. Some people in the know about this would look out for the cameras, then deliberately set off the edge and wave cheerfully at the camera. Guess it gives the guy reviewing the footage a nice highlight.

Edgemaster

When leaning on the door when stationary, the circuit won't be triggered, it is as the train begins to accelerate that the leaning person pushes the door slightly open, which then trigger the cutout.
The differences between trains depends on the strength of the springs that hold the doors shut - some doors open less easily than others.

Leytonstoner

It's quite sensitive on the 1992 Central Line stock, amplified by the brisk acceleration which encourages passengers to steady themselves by pressing on the doors. The resulting braking is then quite abrupt which must really tick the train operators off.

hostile_17

Interesting article... I knew the general reason of what was going on, but not the nuances. Angry drivers though.. boo hoo. It's hardly like we WANT to be crushed in!