I guess I'm confused by why non track -based solutions are necessary? ETCS L2 would gracefully handle a system restart and automatically force a 40kph speed limit until a train traversed the unknown track proving no occupancy, and allowing proceeding trains to go faster. This all works on a track-based system.
Because railway infrastucture companies would like to not pay for them, not build more tracks and still get more capacity due to "digitization".