Monument to Armagh rail disaster

A good book on the accident is J.R.L.Currie's "The Runaway Train: Armagh 1889". There were so many things that went wrong in the operation of the trains on that day. The overloading of a train in relation to the locomotive power, the locking of carriage doors that meant escape was impossible for most, the dividing of a train on an incline, the fixed time operation that meant a second train was allowed follow the special. Fascinating for the student of railway accidents but not so damn amusing if you were caught up in it.

that's what I said...

to spell it out to casual readers,

the train was fitted with non-failsafe vacuum brakes. In other words , the loco sucking the air out of the vacuum brake system applied the brakes... uncouple a carriage, and thus the brake pipe, and you release the brakes throughout the train.

This is "simple vacuum braking" . In automatic vacuum brakes, the brakes are released by the vacuum, therefore if you destroy the vacuum, you apply the brakes.

The officials on the ground that day uncoupled part of the train on a steep gradient after the train stalled. The Guard's handbrake (and some stones placed under wheels (!) ) failed to hold the train and it rolled back into the following train which had been allowed to follow the first after a time interval

As a direct result of this tragedy, automatic brakes were made compulsory and a system was made mandatory where (basically) a train had to clear a section before another was allowed to follow. A milestone in railway safety in Britain and Ireland