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a release button on his instrument, which allowed a tablet (a metal plate inscribed with the name of the section) to be withdrawn from the caller's instrument.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 Until the tablet was replaced in one of the instruments, another tablet could not be withdrawn from either of them.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.