If each train was fitted with a device that transmits a unique RFID pulse and each signal (and/or other trackside objects) fitted with devices to receive these pulses, isn't that a more resilient and accurate way of tracking trains?
ETCS▸ does that, but differently. It relies on the train knowing where it is along the track, and communicating with network control. It uses several sources of position data, because using any single system is a risk. In the case of ETCS it's GNSS (GPS etc.),
balises in the track, and a high integrity odometer.
Of course ETCS doesn't use the latest technology - this is the railway! And one known gap in all such systems to date is being unable to directly check which track you are on, except at a
balise. I think that leads to extra
balises being needed - and they are expensive to install. I think adding new sensors to ETCS is in theory quite possible, though the bureaucracy involved might make it a lot less so. Though note that anything installed in large numbers on the track, even if you think it is inherently cheap, has a high cost both initially and in maintenance (including checking its presence and operation).