The Abermule train collision was a head-on collision which occurred at Abermule, Montgomeryshire, Wales on Wednesday 26 January 1921, killing 17 people. The crash arose from misunderstandings between staff which effectively over-rode the safe operation of the Electric Train Tablet system protecting the single line. A train departed carrying the wrong tablet for the section it was entering and collided with a train coming the other way. Background The Cambrian Railways, which traversed Wales from Whitchurch in Shropshire to and , via , contained a number of single line sections. The small station of Abermule was a crossing station between two such sections. To the east was , to the west was Newtown. To protect the single line sections, Tyer's Electric Train Tablet apparatus was used. Two linked tablet instruments were used on each section, one at each end. To allow a train to proceed into the section, a call button would be pressed on one instrument, alerting the operator at the other end of the section. If the other operator was in a position to accept the train, he would then press a release button on his instrument, which allowed a tablet (a metal plate inscribed with the name of the section) to be withdrawn from the caller's instrument. The tablet would then be placed inside a pouch fitted with a metal loop (which allowed it to be easily picked up or handed over by a train crew while in motion) and given to the driver of the train as proof of his authority to occupy the section. Until the tablet was replaced in one of the instruments, another tablet could not be withdrawn from either of them. Tablets of adjacent sections had differently-shaped and -positioned holes and notches in them to prevent a tablet being inserted into wrong instrument. This system had protected the Cambrian Railways for many years. There was a weakness at Abermule, in that the electric tablet machines and the other block telegraph instruments were kept in the main station buildings, while the signals were worked from a separate signal box at the east end of the station, and some of the points from a ground frame at the other end of the station. Regulations specified that only the stationmaster or signalman were to work the tablet machines, but it was common for both to be occupied with duties away from the station buildings, and