LZB uses cable loops.

ETCS can use cable loops, but for full functionality a radio data connection is required.

Yeah, generally that’s true - it’s certainly not a long-range system. I’m not clear on what the actual problem is at Stratford, but I do know these systems can be quite sensitive and it can be tricky to ensure they operate reliably alongside each other.

In the case of Crossrail the core (new tunnels) use CBTC signalling, I think based on Siemens TrainGuard - I think the specification of the project predates ETCS being proven as a viable solution for this particular kind of service. The services leaving Stratford to the East use TPWS/AWS in common with most services on the UK mainline network.

The challenge is in the transition between the two systems, as I understand it - anything out of the ordinary and the train will of course come to an immediate halt.

It’s an interesting idea and not without some merit. Some signalling systems do actually use the rails as a conductor already, although that’s just basically “is there are train on me or not” detection.

I’m getting out of my depth, but I’m sure I’ve seen at least one signalling system that uses the track circuit as a data signalling channel to the train to provide in-cab signalling, so I’d guess it’s at least theoretically possible.

I’d imagine it would have much the same challenges as radio in practice though - interference from poor connections and ambient noise, not to mention your channel is basically a massive antenna.