How to find abandoned tracks?

I would like to build a vehicle that is either pedal powered or possibly powered with a small engine that will travel on RR tracks. Think of a bike that rides on the rails.

Obviously I don't want to ride on tracks that are in use. So, where can I find a map (or other form of description) of railway lines that indicate which ones are abandoned?

I've looked on the net and can't find anything that's very detailed and nothing showing which lines are abandoned. I know this is illegal so you don't need to alert me to this fact. I live on Northern Ohio (USA) if that makes any difference

Although many miles of rail, especially today, are salvaged upon being abandoned, many remain in place for several years, sometimes decades (when rail lines are abandoned and sold the property, if not purchased, reverts back to private ownership, over which the rail line passed through). Now, finding these lines, on the other hand, can be a real challenge. There is no real way to find "mainstream" sources of information on abandoned lines although DeLorme's state atlas' and gazetteers used to include many, and distinguished them from active rail lines by coloring them light gray.

Also, if you type "abandoned railroads" into a search engine like Google you can get some pretty good information on defunct lines. Probably one of the best that covers abandoned lines nation-wide is AbandonedRailroads.com. In any event, good luck!

Rail Roads don't usually abandon lines, they either sell them to another railroad or salvage the materials. Tennessee has a large mining comunity, and when the mines play out, they pull up every bit of usably rail, the ties, and the ballast. The only thing left behind is the dirt embankment.
About the only abandon rails I know of are on State, City, or Federal land. Usually old rail spurs dating back to World War 2 or downtown lines that are in the streets and can't be torn out easily.
I imagine there are abandon lines somewhere, but due to property liability, no one will let you use them, and won't tell you where they are.

We call those "flying shingles", and we call that "bandit running". Why do that when you can do it safe and legit?

You do know there are large clubs of people who do this very thing. http://www.narcoa.org/ They're good folks who have a lot of fun, but they have zero tolerance of bandit runners. So you're either in or you're out!

There are also a few folks who have informal arrangements with railroad companies to "help keep their disused lines passable" which means clearing brush you'd need to clear anyway.

There are also many, like myself, who volunteer at railway museums and part of our activities are "inspecting" the railroad lines. Museums that come to mind are Cuyahoga Valley Line, Toledo Lake Erie & Western, Little River Railroad, Adrian & Blissfield Rail Road, Southern Michigan Railroad, just to name a few.

inn the back of trains magazine the classified section has all kind of maps and related things

Yea I would not know I am not really into all of this stuff...