In most book-to-screen transfers, the big question is what to eliminate. But with Chris Van Allsburg’s popular children’s book “The Polar Express,” the challenge was what to embellish.

“I finally agreed to do the film when I came to the realization that we should use the book as an outline of the movie,” director Robert Zemeckis said in a telephone conversation last week.

“Our rule was to include everything that had been in the book and to add to it. Never subtract from it.”

Zemeckis broke new ground in special effects with “Forrest Gump” and “Who Framed Roger Rabbit.” He seemed a natural to direct “The Polar Express.” But when Tom Hanks offered him the deal, he stalled.

“Tom always owned the rights to the book, and he loves that book,” Zemeckis says. “I love it, too. But it’s a small book with enchanting drawings. I was worried about it just being too slim to transfer to the screen.”

The book tells the story of a young boy who’s starting to doubt the existence of Santa Claus.

On Christmas Eve, a locomotive pulls up to his house, and he hops aboard for a trip to the North Pole.

The film adds several characters, including a philosophical hobo, a sad-eyed boy from the wrong side of the tracks and a black girl of strong leadership abilities.

“From the start, we just wrote the girl as an African-American,” Zemeckis says. “We weren’t planning any sociological message. We just thought it was a nice touch.”

Each scene in the movie is clearly inspired by the drawings in Van Allsburg’s book, which in turn were inspired by 19th-century German artist Caspar David Friedrich. One of the film’s most elaborate expansions occurs when Polar Express waiters serve dinner to the children heading north for a look at Santa in action. It turns into a musical production number. In the book, the waiters simply passed out hot chocolate with no fancy footwork.

“The idea of a Busby Berkeley musical number just came to me,” says Zemeckis, referring to the choreographer of such vintage Hollywood musicals as “42nd Street” and “Footlight Parade.”

“It was a challenge, and I just wanted to see if it would work. It did.”

Advance word on The Polar Express is largely positive. But it faces competition for the family trade, opening five days after the well-reviewed “The Incredibles” and nine days before “The SpongeBob SquarePants Movie.”

“Our only competition is bad movies,” Zemeckis says. “We’re not in the refrigerator business, where the customer just needs one refrigerator. Moviegoers can see as many movies as they want. If they see a good one, it makes them want to see more. If they see a bad one, it keeps them away.”