cool things with wheels, since 2006
Build trails across the country like this https://en.wikipedia.org/wiki/Trans_Canada_Trail
that is a fantastic idea! There isn't a bike path worth mentioning around or through the USA, and Canada's is only 1/2 finished, and 1/2 existing roads.
The Fox River Trail in Northern Illinois, west of Chicago, covers 40 plus miles, most of it on repurposed interurban RR beds along the Fox River. Easily accessible and a beautiful ride along the entire length. Numerous times I have ridden along one of those very gradual uphill sections and offered silent praise to the long forgotten surveyor who laid out that RR line!