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BT built a system of underground copper pipes/waveguides for microwave transmission in the 1970's - it never got beyond a testing platform in Suffolk (Martlesham Heath to Wickham Market) due to fibre optics becoming the clear winner.

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AT&T built one as well, the WT4/WT4A system.

https://archive.org/details/bstj56-10-1829

And some above ground waveguides alongside a main road near Ipswich - not sure if they are still there.

There's some cool but not very ready tech called surface wave transmission lines that use horn like "launchers" to establish surface waves on single conductors like power lines. They have very low RF losses up to the tens of GHz as long as stuff stays a few wavelengths away from the lines.

In way, if you try hard enough, most powerlines could act like inside out waveguides for high data rate RF backhaul.

There is a story that the "Radio Tower" at BT Labs was built as a waveguide - this is on the A1214. Given that the office accomodation in it is so weirdly laid out I can believe that this might be so.

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