Balise

A balise /bəˈliːz/ is an electronic beacon or transponder placed between the rails of a railway as part of an automatic train protection (ATP) system. The French word "" is used to distinguish these beacons from other kinds of beacons.

Balises are used in the KVB signalling system installed on main lines of the French railway network, other than the high-speed Lignes à Grande Vitesse.

Balises constitute an integral part of the European Train Control System, where they serve as "beacons" giving the exact location of a train. The ETCS signalling system is gradually being introduced on railways throughout the European Union.

Balises are also used in the Chinese Train Control System versions CTCS-2 and CTCS-3 installed on high-speed rail lines in China, which is based on the European Train Control System.

A balise which complies with the European Train Control System specification is called a Eurobalise.

A balise typically needs no power source. In response to radio frequency energy broadcast by a Balise Transmission Module mounted under a passing train, the balise either transmits information to the train (uplink) or receives information from the train (downlink, although this function is rarely used). The transmission rate of Eurobalises is sufficient for a complete 'telegram' to be received by a train passing at any speed up to 500 km/h.

A balise may be either a 'Fixed Data Balise,' or 'Fixed Balise' for short, transmitting the same data to every train, or a 'Transparent Data Balise' which transmits variable data, also called a 'Switchable' or 'Controllable Balise'. (Note that the word 'fixed' refers to the information transmitted by the balise, not to its physical location. All balises are immobile).

...
Wikipedia