I have a chest minecart on a loop servicing an ice farm.  It run on a loop and triggers a bunch of pistons to push the ice.  It works really well, except, the minecart disappears pretty much every time I exit the game and come back in.  I've replaced the thing about 10 times at this point.  I've seen a couple references to similar issues, but some are other versions of the game, and others, there doesn't seem to be a solid solution.