Trains moving with doors open

All passenger doors on the train, the M doors (doors in the middle of the cab that open out onto the outside world) and possibly some others are part of an interlock. When all doors are locked and proved closed, one or more lights (the so-called pilot lights, or Doors Closed Visuals (DCVs)) will come on in the cab and the train will be able to depart. If all of the interlocked doors are not proved closed, you will not have an interlock and you will not get a brake release. The train won't depart.

If the train does depart and then one of the formerly closed doors should come open, you will lose the interlock, and the pilot light with it, and the motors should drop out. If the interlock is not re-established the train should be stopped by the train operator.

Now, there is something called the round train circuit, which includes the interlock. If the round train circuit is cut out then the train can depart with doors open - however a train should not be in passenger service with its round train circuit cut out.

The next bit is where different stocks come in. On older trains, the doors are operated by compressed air and held shut by air pressure. They are designed to have a bit of give so that a bit of sleeve or something that might get caught in the doors can be freed. But as the door opens more and more it becomes harder and harder to overcome the air pressure, so the doors can only open a little. If the doors are forced to open too far by these means, the interlock will be lost, the motors will cut out. But there is a little lee way.

When they were designing the 2009 stock, they found that the doors were too heavy to be held shut by this means and that they would come open under their own weight on heavy acceleration and braking. So on the 2009 stock and newer the doors are electrically operated and do not have this give. Instead there is a sensitive edge system - as well as obstacle detection to prevent the doors closing on anything in the first place - which detects if something is caught in the doors. This was famously temperamental when first introduced and caused numerous delays, but it's settling in now.

That's how it's supposed to work. Obviously what we had on the Piccadilly line recently looked at first glance like a fault with this important safety system (I believe the cause has been established as an isolated operation of the outside door operation cock), which is why people were concerned, because a train shouldn't be able to motor without all its passenger doors closed and an indication that doors have come open should be given at once to the driver.

P.S.: Expect variations in specifics between stocks. For example, the Northern and Jubilee line trains will not move if either the nearside, or offside cab door is open, because of ATO (concerns that a train may take off without its driver, I think). The same is true of Central line 1992 stock trains in Auto, though not in Coded Manual and I think it's true of S stock as well.

On older stock, there was nothing to stop the driver from moving off with the doors open. However, the guard could not give the start signal to the driver until all doors were closed and the pilot light came on.

If you're interested in the topic, there's a lot of good information in the RAIB report into the incident at Holborn on the Piccadilly line, where a woman got dragged a short way by her scarf. And also good stuff in their report into an incident at Warren Street on the Victoria line, where a train did depart with its doors open.

Ooooh thanks for the corrections Dunno why I thought the round train was related to the doors, ta for clearing that up

On one point though, I think things may be different on the 1992 stock. Certainly, the impression I get from one of aslefshrugged's blog posts is that a 92 won't move off from a stop without an interlock.

Ahhh fair enough - that must be the source of my misunderstanding. Thank you

The M door on the D stock or on a C stock or even A stock can't speak for others though is not part of the interlocking for the doors I have had it open many times on various faults usually traction earth faults to see if you can see anything which is causing the fault of course for a bit of safety this is done and a slow speed. Now with thermal cameras being a lot more reliable this should no longer be the norm.

Dunno, even the newest (mega-expensive!) thermal cameras allegedly don't work through glass.

Add 95 stock to the list where the M door isn't on any kind of interlock for manual driving, ATO is different for obvious reasons, I'm not sure if this is just the active M door or the other end too for ATO working. There is an intruder alarm as well such that an open M door at the other end of the train would be indicated to the driver, activation of this would be investigated as a matter of importance.

Odd, since you can certainly feel the heat of the sun through glass. Actually, they do work, because IR film and IR enabled sensors both work with glass lenses. If heat couldn't get through glass, we wouldn't need double glazing!

It brings to mind a journey that involved passing some trackside maintenance where they had fires burning wood every few dozen metres.

It was, I think, a unique experience to feel a regular series of extremely short pulses of heat as the train sped past. (You can feel variations in heat radiant heat passing trees that are blocking the sun, but you don't get the same, regular long off, very short on, effect.

I can remember travelling on a " Red Hen " train in South Australia in the early 90's they could operate with the passenger doors open. Can't remember though if it was a sliding door or a slam door I'm sure it was a sliding door.

Thought that was banned now but I know the way they get round it so the door looks like its closed