What is the function of balises in an ATP system?

In an ATP (Automatic Train Protection) system, balises function as trackside transponders that transmit information to the train about track conditions, speed limits, signal aspects, and other relevant data to ensure safe train operation. A balise is a device installed between the rails that communicates with the train as it passes over. There are two main types of balises: fixed balises, which transmit the same information to every train, and switchable balises, which can transmit different information depending on the state of the signaling system. The information transmitted by the balise is received by an antenna mounted on the underside of the train. The onboard ATP system then uses this information to monitor the train's speed and position and to enforce speed limits and signal indications. For example, a balise located before a curve might transmit information about the speed limit for that curve. The onboard ATP system would then ensure that the train does not exceed that speed limit. If the train exceeds the speed limit or approaches a signal at danger (red) without appropriate braking, the ATP system will automatically apply the brakes to stop the train before it reaches a point where a collision or other hazardous situation could occur. Balises provide a reliable and accurate way of transmitting critical safety information to the train, helping to prevent accidents caused by human error or equipment failure. They are a key component of many modern ATP systems.