Basically, it's a lot like a minecart. You place it on a track and you get on it. However, it has an alternative power-source, the player. You build momentum by pressing a button, the more you press the button the more speed you gain. Why? As an alternative method of travel. Minecarts really aren't a reliable method of travel without abusing glitches or using mods. And ya... this would be a mod but I think it'd feel more like part of the game instead of a mod, and maybe even convince Notch to add something like it in the future.
If you need more details or need graphics, I can do that.