Most older (BR era) stock including 158's etc will have a direct brake application when the 'Pass-Comm' (Passenger Communication) is pulled. There is no facility to speak to the traincrew on these trains so it's a case of going for a wander to see what's amiss! The orange hazard light outside will illuminate on whichever coach is affected, which helps narrow it down, although there will still be several levers in each car. As said above it isn't an immediate stop as that isn't really possible on a train, just a general feeling of deceleration and then a slightly abrupt stop, as the Driver can't east off the brake with any type of emergency application, as they would usually. Not sure what happens on the newer stock where the Driver can speak to the Pax and override it, but on the 'old school' units the Guard/Driver simply resets the lever with a carriage key, brakes then come straight off. If for any reason it causes major issues the train can be moved by overriding some safety systems, but not in general service.
In my experience most Drivers are helpful and you generally end up both walking through and meeting in the middle somewhere, which makes it quicker. I've had a few incidents of it being operated, including once in Fisherton Tunnel at Salisbury, but never for a genuine emergency. It's usually either some idiot who thinks it's funny to delay everyone (including themselves!) or quite often on 158's it'll be somebody trying to flush the toilet or open the toilet door! :roll:
