I know of no instance of this occurring with a viable line. Because of the boom-and-bust cycle of mining in the Intermountain West, rail shortlines were frequently abandoned. Rails could be scavenged for use elsewhere - and this typically happened whether it was by the owner of the shortline or by someone else, but one would not call this theft, necessarily. It was "opportunistic recycling" and no one was likely to object.

Rails were extremely heavy, and steel was not particularly valuable - certainly not enough to justify the effort of pulling it up, transporting it, and then attempting to "fence" the material. There were better ways to make a living, legal or not.