Balise / Eurobalise
A balise is a passive transponder mounted between the rails that transmits a short data telegram to a passing train. In ETCS, Eurobalises give the onboard unit a precise location reference and, in Level 1, carry movement authority and speed information from the lineside signals to the train.
How it works
The balise has no power supply of its own. As the train passes, an antenna under the vehicle energises it with a 27 MHz signal and the balise responds with a telegram on 4.23 MHz. Fixed balises always send the same telegram, typically a location identifier and track data. Controllable balises are connected to a Lineside Electronic Unit (LEU), which changes the telegram according to the state of the signal or interlocking.
Balises are installed in groups so the train can determine its direction of travel and detect a missing or faulty unit. In ETCS Level 2 and 3 they serve mainly as location references that reset the onboard odometry error; the movement authority itself arrives by radio from the Radio Block Centre.
Why it matters for security
Balise telegrams are protected against transmission errors but not against deliberate manipulation, and the devices are physically accessible along thousands of kilometres of track. Research has shown that forged or replayed telegrams can mislead a train about its position or the permitted speed, which is why the onboard unit cross-checks balise data against other sources and why linking information from the RBC is used to detect a missing or unexpected balise.
The LEUs that drive controllable balises are increasingly connected to interlockings over IP, which extends the attack surface from the trackside device to the signaling network behind it.
Related solution
Operating ERTMS across national rail networks
Visibility into the trackside and radio interfaces that carry ETCS data to the train.
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