In Eastern Washington State I know of at least two spur lines that were abandoned and the right of way converted into trails. One of them is over 50 miles long and both are especially popular with horse riders...
I also know an abandoned line that was converted into a roadway for logging trucks and sportsmen near Avery, Idaho in the Rocky Mountains. There are two tunnels that you are required to have a CB radio and check oncomming traffic before entering. Pretty cool stuff. It was the very best way to get to my favorite fishing place on earth...
Now that sounds pretty cool.
I never got to Idaho unfortunately, but I did go through the Rockies...well I must have when I was in Colorado. Loved Colorado....Loved the US, soooo many gorgeous and totally unique places everywhere...