The question is whether a standard bicycle with standard tires can be ridden for long distances by staying on top of and following a standard railroad track?
I’ve never seen it done and wonder if it is even possible.
Check out this YouTube video, starting around 1:00. The guy is doing stunts on railroad trucks, but riding faster should be easier.
Most people don’t have the skills to do this, of course. A bicycle is primarily balanced by steering input. But if you can do a track stand (i.e. balancing a bike without moving), I think you can ride on an arbitrarily narrow path.
I’ve seen someone who has hooked up a strut thing with a guided wheel set up to his bike (think like an outrigger canoe) who rides on the old tracks around here. I think without such a thing it would be insanely difficult-- the tracks are slightly convex. I can’t even stay on a white line for very long.
Your question has no definitive answer. Can it be done? Of course. How far? It’s like asking how far an acrobat can walk on a tight rope. There are some pretty amazing bicycle acrobats that could probably go a fair way, but eventually they would probably fall off at some point.