What is TERN?

TERN (which is an acronym for Token Exchange using Random Numbers) is a system that reinforces verbal movement authorities given out by a train controller.

It was was first developed about 25 years ago as a simple means of allocating possession tokens to remote track maintenance gangs. Over the years, the huge advances in technology have enabled TERN to become akin to a signalling system but still retaining the original ethos of being very simple.

Because the security of the system is built into the individual units there is no need for communications links to be secure. Safety critical information is encrypted for transmission, but no authorities are issued or rescinded on receipt of these transmissions alone. Further exchanges of checksums are required before there are any changes to indicated authorities. The present form of TERN uses IP technology and instant messaging software to transmit data.

TERN will work well if GPRS coverage exists or if a digital back-to-back radio system is available. 

There is an audio version of TERN that uses analogue radio, although this is suited to very simple applications.