Some of the London Underground tube trains have buttons to open the doors, notably the Jubilee, Northern and Central lines. As all the doors on every carriage always open with no user intervention at each stop, some Londoners have speculated as to why these buttons are there, and also why they light up at every stop enticing you to press them - which appears to do absolutely nothing at all. As a hardened London commuter of many years standing, using the tube every day is only bearable due to the understanding that we commuters have of how to use the system. Unfortunately, many tourists do not know or understand these rules and it can help greatly if Londoners can anticipate some of the things tourists regularly do, like stepping off a train and stopping dead to look around, doing the same at the foot of escalators (usually with a huge bag in tow - though I will concede the bag is a bit of a warning itself), walking in through the out passage, being totally unable to tell left from right, walking right up to a ticket gate (blocking it) THEN start searching for their ticket (aw, come on, they've only had 15mins sitting in an underground train car to prepare for the exit, give 'em a break), being completely unable to work out which way up the ticket goes into the ticket barrier, etc etc. Well, I discovered this button lurking on a Jubilee line train. I now think they just forgot to add the instructions on most of them. So now I know what the buttons are for. Prepare for evasive action, everybody!