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Yeah, I rode part of the Transubayenne mtb route in the Ubaye valley on a road bike this summer, liked it a lot. I heard it was built once upon a time for a railway, but never finished. Nice gravel, mountain views, dark tunnels, small new bridges. Rough stuff on a roadbike, but ridable. There was water streaming on the ground for the whole width of the tunnel for a few hundred meters in one of them, but it's usually dry, I've heard.
Ciclovia Alpe-Adria Radweg/ Via Treviso / Fvg 1 or what ever you should call from Italy to Austria was interesting too. Smooth asphalt, old abandoned small buildings, villages, an old bridge made of steel. The tunnels have lights in them. Many of us used this in TCR last year.
There's a nice quiet one in Finland too, mosty gravel, from Hyvinkää to Karkkila .
Don't anything about the ones in UK though, but interesting thread.
I know this is probably mostly aimed at UK routes but the French are great at converting disused railway lines to cycle routes. Voie Verte des Gaves in the Pyrenees is my favourite and the Avenue Verte makes getting to Paris Pretty easy too.