Laura Craft trembled with excitement as she climbed aboard Texas Tornado, the new roller coaster at Six Flags AstroWorld in Houston. The 50-year-old lawyer wasn't about to miss an amusement billed as "one of the steepest coasters on earth."

The initial 112-foot climb went fine. But on the way down -- a hair-raising 70-degree drop -- Ms. Craft's body slammed into her seat, and she felt a sudden twist in her back. And that led to yet another scary ride: an ambulance trip to a nearby emergency room.

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