The term used when someone delays a train with the alarm is passenger action. The alarm handles on tube carriages are provided for emergencies. Apart from invoking a penalty fine, people often wonder what else happens when an alarm is pulled.
On the 1920s stock, the alarm handle is attached to the hydraulic braking system. Pulling the handle causes the brakes to come on. The brakes will only come off when the handle is reset and the air compressor run.
One refinement to the alarm handle was adding a lock. This means that an allen key is required to reset the alarm.
By 1990, the old hydraulic mechanisms on all trains were replaced with electronic alarms, which merely alert the driver. These alarms have a microphone and speaker, allowing two way communication with the driver. As far as I know, there is still a physical lock which requires resetting with an allen key, but there is no reason why this could not be replaced with electronics.
A personal encounter
IvorW encounters a phenomenon in the morning rush hour.
Having boarded a Central Line train at North Acton bound towards Central London, I find myself in a somewhat empty part of the carriage. I am sitting opposite a seat occupied by a makeup bag. There are two women seated at the other end of the bank of 5 seats I am sitting in, and no others in the opposite bank.
I ask my fellow passengers about the bag, and none of them owns it. The two women are carrying on discussing the bag. The Irish one sitting nearest me is quite nervous about the situation, and is calmed down by the other woman who wants to kick the bag off at the next station.
Nothing is kicked off at East Acton Station, and I decide to act, telling the two women that I will do something about the bag. As the tube draws into White City Station, I am standing up, and I pull the alarm when the train doors are open. This results in a sunken panel lighting up Driver Aware in red. Knowing that the assembly has a microphone, I announce that there is an unattended bag in the carriage.
Perhaps because I have made this announcement, there is no alarm sounded. Next, two uniformed LT staff members enter the carriage, and I indicate the unattended bag. All passengers are evacuated from the carriage, the bag is dealt with, and we all reboard within 2 minutes. I realise that this is a drill that the LT staff have performed many times in the course of their duty.
After this incident, I am left wondering how many members of the public would have done 'the right thing'.