Big Thunder Mountain Railroad is an excellent roller coaster in Frontierland and one of the best attractions at Walt Disney World's Magic Kingdom. The minimum height to ride is 40 inches.
If a fear of heights has kept you (or your kids) from trying roller coasters, this might be the ride for you. You'll ride in mine train cars in and on Big Thunder Mountain, home to an abandoned gold mine. It's the "Wildest Ride in the West," according to the spiel at the loading station, but Thunder's actually one of the milder roller coasters around. Since you're riding on the mountain, you're rarely more than a few feet off "the ground." Still, the ride tops out at 32 miles per hour, so it's got enough oomph to reward a junior thrill-seeker-in-training.
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