I was interested in the same thing a while back, I pictured rail biking hundred miles of abandoned lines through the countryside. So then I did a search for abandoned rail lines here in my home state of Kentucky. Sure enough they had a lot of them mapped out. Problem was that they were three miles here, seven miles there, none very long and the condition of the tracks is awful. All overgrown with fallen trees across the tracks everywhere. But...... if your area has a long unobstructed, unused line, I think it would be great.