The Open Guide to London: the free London guide - Differences between Version 2 and Version 1 of Tube/Alarm

Contents are identical

Passenger Action

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.

Old trains

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.

Modern trains

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.