The train crested the mountain, then tipped downward, rushing like a roller coaster. And, as if they were on a roller coaster, many of the children seated inside raised their arms, and some squealed.

Of course, they were all sitting in a room, inside a building at SeaWorld Orlando, rocking up and down, back and forth or side to side, yet going nowhere while watching a movie. They were experiencing SeaWorld's newest attraction, the Polar Express Experience, taking a wild but simulated train ride to Santa Town.

As recently as mid-November, riders in that same room were taking a much different ride, a simulated helicopter flight over the edge of a cliff somewhere in the Arctic. And a couple of weeks from now, the helicopter ride will be back.

SeaWorld has developed what many in the industry have been expecting to see in a major theme park for some time: a quick-change conversion package for a high-tech simulator ride.

All it took was acquiring the rights to a theme-park version of the movie The Polar Express, investing more than 100 hours in engineering and computer programming and building sets based on the film.

"I'm not going to say it's easy, but we have a phenomenal team, from the art direction to the programming of the ride to the visuals," said Michael Fletcher, SeaWorld's vice president for entertainment.

Other theme-park companies have swapped out simulator-ride films and changed themes before; Universal Studios is doing that now with its old Back to the Future: The Ride attraction, to create a new ride based on The Simpsons TV show. And there is always industry talk of new possibilities, such as whether Walt Disney World would replace at least part of its Star Tours simulated spaceship ride with a new Star Wars-based pod-racer experience.

But SeaWorld's changeover is different because it's temporary and was swiftly executed. In a matter of weeks, the park switched off its regular attraction and replaced it with what its new general manager, Dan Brown, calls a new holiday-season centerpiece.

"I'm intrigued by the idea," said Kevin Coons of Gary Goddard Entertainment, a California-based attractions-design firm that helped develop simulator rides such as The Amazing Adventures of Spider-Man at Universal Orlando's Islands of Adventure. "I think it's a smart way to go."

A new simulator ride would cost tens of millions of dollars to build. A new software package that creates a new experience -- converting an arctic helicopter flight into a polar train ride, for example -- costs a small fraction of that amount.

"If it is successful, you'll see an expansion of that sort of concept to other attractions," said Dennis Speigel, president of International Theme Park Services in Cincinnati.

The Polar Express Experience didn't start out as a ride.

Working with Warner Bros. Pictures and Shangri-La Entertainment, which produced The Polar Express movie, California-based attractions company Iwerks Entertainment had created a short version of the 2004 animated feature for possible use as Christmas-season entertainment in theme parks.

Meanwhile, Busch Entertainment Corp., which operates SeaWorld, was looking for a new holiday attraction.

Mike Frueh, Iwerks' vice president and general manager, said he and others at Iwerks were not thinking of the film as the basis of a simulator ride until Busch suggested Wild Arctic as a platform.

"The Busch team, they're visionaries and they're quite creative," Frueh said. "We've had a long relationship with Busch in different roles, and had mentioned the film to them. It kind of evolved from there."

SeaWorld's Fletcher said the film was an obvious choice for the ride.

Engineers took Iwerk's film and used hand controls similar to video-game joysticks to program new movements into the Wild Arctic simulators to accompany small sections of the film. Once the motions were synchronized with the film's movements, the package was fine-tuned on a computer to create a seamless ride.

"When you're working on a project like this, it's always a joy to work with an idea that is a perfect fit," Fletcher said.