Eurobalise

An Eurobalise is a specific variant of a balise being a transponder placed between the rails of a railway. These balises constitute an integral part of the European Train Control System, where they serve as "beacons" giving the exact location of a train as well as transmitting signalling information in a digital telegram to the train.

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 transmits information to the train ('Uplink'). The provisions for Eurobalises to receive information from the train ('Downlink') has been removed from the specification. The transmission rate is sufficient to transmit at least 3 copies of a 'telegram' to be received by a train passing at any speed up to 500 km/h.

Eurobalises are typically placed in pairs on two sleepers in the center of the track. For ETCS they are typically spaced 3 metres apart. With the balises being numbered the train will know whether it travels in nominal (1→2) or reverse direction (2→1). Singular balises exist only when linked to a previous balise group or when their function is reduced to provide only the exact position. There may be up to 8 balises in an balise group.

Balises are differentiated as being either a 'Fixed Data Balise' 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).

A 'Fixed Data Balise', or short 'fixed balise' is programmed to transmit the same data to every train. Information transmitted by a fixed balise typically includes: the location of the balise; the geometry of the line, such as curves and gradients; and any speed restrictions. The programming is performed using a wireless programming device. Thus a fixed balise can notify a train of its exact location, and the distance to the next signal, and can warn of any speed restrictions.

A 'Transparent Data Balise', or short 'controllable balise' is connected to a Lineside Electronics Unit (LEU), which transmits dynamic data to the train, such as signal indications. Balises forming part of an ETCS Level 1 signalling system employ this capability. The LEU integrates with the conventional (national) signal system either by connecting to the lineside railway signal or to the signalling control tower.

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