I recently stumbled across a StackExchange question asking about the electronics involved with single-wire data transfer. I pointed them in the direction of the BCRA's CREG group, and highlighted Brian Pease's "Data Transmission Through a Long Single-Wire Phone Line" but don't know enough to actually answer their question. All the other answers are workarounds such as using twisted pairs, optical fibers etc, so I thought I'd highlight it here so that someone more knowledgable can weigh-in with an actual answer to their question!

Try to get a message to,
"Shropshire Caving and mining club." They have used single wire extensively.
https://www.shropshirecmc.org.uk/radio.html