Continuous and Intermittent ATP
ATP systems may be broadly grouped as continuous and intermittent. With continuous ATP, a cable is laid between the rails for the full length of the block section. The rails themselves may also be used as the cable whereby the track talks to the train. With intermittent ATP, beacons called balises are mounted between the rails on the approach to signals, and perhaps a few other locations.
ERTMS is an attempt to set a standard for Mainline ATP across Europe where balises, GSM-R Radio and on train equipment made by any manufacturer (who are part of the working group) work together with each other. This is achieved by carefully agreeing the functional specification of the system and the format and transmission methods of data across the air gap, both transponder and radio (GSM-R is the most common system in use).
Read more about this topic: Automatic Train Protection
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