Unbrakeable minecart (fallen)

Drop a minecart down a hole so its lands on a hopper, before that place tripwirehooks and string like the picture. have a piston push a full block so it brakes the string and not the minecart and tada you have made an unbrakeable minecart. (it wont even mover the stoneblock behind the minecart as it should be and do if you place a rail and minecart on it.