THEIR FACES SUM UP the thrill ride experience perfectly: joyous abandon and sheer terror. There was plenty of both to go around- quite literally, in the case of the looping FLYING CARS-at Riverview (1904-67), once the largest amusement park in the world. Also one of the safest, but that was a relative distinction. In the early '50s it was boasted that RIVERVIEW had gone seven years without a fatality.