Rail Grief?

[TehCthulhu]

I recently realized i have no idea what a rail grief actually is. I've seen various changes to the topography on Ruenge's netherrail, and since none of them keep you from being able to use it, I never bothered mentioning it. So now, out of curiosity, I am wondering: is a rail grief limited to picking up the tracks, anything that keeps people from using the rail system, or any movement of blocks around the rail?