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.