A small segment has been under proposal here in Oregon for a number of years.
https://www.pdxmonthly.com/articles...-coast-salmonberry-trail-gets-the-green-light
Although abandoned rail ways make for great routes due to their original grade requirements being favorable to cyclists there are many issues, #1 being funding, to restoring them for public use.
As mentioned even ability to maintain existing sections, there is one here called the CZ trail that falls into this category, that in fact are that way due to seeing little use quite frankly and as we all know the squeaky wheel gets the grease.
As much as I would like to see more RTT conversions that could possibly make a coast to coast ride possible it is certainly not going to happen unless it gets a large amount of financial backing.
Unfortunately here in the US unless there is a fear factor involved for large projects a fitness factor will never get the job done.
Maybe the great grand kids will be able to ride it on their hover bikes.