Rail riding looks like an activity that would get old quickly. It involves modifying a bicycle (or building a kind of go-kart) so it can ride along train tracks. Once you get past the novelty of motoring along rails meant for trains, I imagine it gets old pretty quickly, not to mention limiting, since you'd be lamenting not being able to go off the rails and travel on the land around like a regular bike can.
But for those fascinated with rail travel, the rail-riding bike might be just the thing to nurture your hobby. If you want a Rail Runner (the go-kart thing), it'll goes for about two grand, and the bikes look easy enough to modify (some video after the jump). Just be sure to stay off the Q line in Brooklyn.