Well, I have a bit of a problem with forcibly converting these abandoned rail lines to bike trails. Often the land was taken in order to build the railways. When the railways fall out of use, the land should revert to private ownership. Now I realize that the people that the land was taken from are long gone, but a government should not be able to take land for one purpose, then later (months, years, decades, it doesn't matter), convert it for another use.
"This is to be built along an unused railway line. I wonder how long this trend will continue - with increased gas prices, at some point somebody will realize that light rail is a useful and fuel-efficient way to move people around the region. "
^^^This is the only reason not to cede unused railways to private ownership. In the future rail will likely be needed for public transportation.