Pulled tail

Pulled tail is the colloquialism referring to the act of a guard or conductor of a railway to apply the emergency brakes when something unexpected has been noticed. This could be an excess of speed in a section of line known to have a lower speed, or strange noises and shaking that might indicate that the train has derailed or something has broken.
In the United States, local colloquialisms include 'pull the air' or 'big hole' as verb phrases describing this same action. In the UK, colloquially the guard would drop the tap and the passenger pull the chain to apply the emergency brake.
Other uses.
In the days before modern electrical communications, applying the emergency brakes briefly was a way the guard could communicate to the driver that (for instance) a conditional stop was required at the next station.
Accidents preventable.
Accidents preventable by pulling the tail: