Thanks for the tag, but it’s quite an old one and found out the answer (and shown how it works) since - it works via Bluetooth.Indeed there is/was, using a wireless communication device, in the same place that Electrostars have their top headlight/line light:
While it at least will have been useful as a testbed for such new technology, it is unfortunately another part on the GWR 769 program that is now a waste - the trackside bailses being another.