What happens when the Alarm Chain (Emergency Chain) is pulled in a coach?
The alarm chain in a passenger coach is designed to create a break in the continuity of the brake pipes (whether vacuum or air brakes), immediately resulting in a loss of brake pressure (or vacuum) and thereby cause the train brakes to be applied. With vacuum brakes, a clappet valve is provided that is released by the pulling of the alarm chain, and with air brakes, there is a similar passenger emergency valve that can vent the brake pipe to the atmosphere. See the page on IR brake systems. At the locomotive, in addition to a warning lamp or buzzer being sounded, in most locos the master controller undergoes auto-regression, with the notches falling to zero rapidly as the locomotive’s motive power is switched off. The guard may also notice the loss of brake pressure (although he may not know it is due to the pulling of the alarm chain) and is expected to apply his brakes as well immediately. It is possible for a driver to override the alarm chain pull in a few circumstances, and this i
