Tyer's Electric Train Tablet
Tyer's Electric Train Tablet system is a form of railway signalling for single line railways used in several countries; it was first devised in Great Britain by engineer Edward Tyer after the Thorpe rail accident of 1874, which left 21 people dead. It was used in New Zealand for close to 100 years until June 1994.
Source: Wikipedia — Tyer's Electric Train Tablet (CC BY-SA 4.0)