“The Polar Express” Needed a Final Magical Song. How 3 Musicians Came Together to Create an Instant Classic, 'Believe' (Exclusive)

When Glen Ballard and Alan Silvestri started working on the music for The Polar Express, they had no idea what the film's final song would be — or how iconic it would become.

The Christmas classic, created by Oscar-winning filmmaker Robert Zemeckis, originated with a 1985 children's book by Chris Van Allsburg, a story that all three of the song's creators, including singer Josh Groban, remember reading themselves or sharing with their children. The animated film, which celebrates its 20th anniversary this year, starred Tom Hanks in multiple roles, namely the conductor, who runs a magical train to the North Pole on Christmas Eve and helps kids rediscover the beauty of belief.

"The whole tone of it was mysterious and I thought, 'Okay, I wanna get on that train with Zemeckis and Silvestri,' " Ballard, a six-time Grammy winner for his work with Michael Jackson, Quincy Jones and Alanis Morissette, tells PEOPLE. "This is not a traditional Christmas book. It has all of the elements of Santa Claus and the North Pole, but it has a dreamlike quality to it, and a kind of mystery to it that I think makes it quite original as a Christmas story."

While the movie was nominated for three Oscars, "Believe," with music by Silvestri, lyrics by Ballard and a vocal performance from Groban, took on a life of its own. The song was nominated for an Academy Award and a Golden Globe and won a Grammy in 2006. Groban even performed the song at the Oscars with Beyoncé the year it was nominated.

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As the filmmaking process began and Ballard and Silvestri — known for scoring films from The Parent Trap to Avengers: Endgame — started making other songs for the movie, including "Rockin' on Top of the World" sung by Steven Tyler, Silvestri also created a light motif. "Alan is always writing themes as we go," Ballard says.

"Believe" came "at the very end of the process," Silvestri says. "We were in the 11th hour, and I remember Zemeckis saying, 'I don't think we have our theme yet.' " After playing around with Silvestri's motif, adding Ballard's lyrics and sealing the song with Groban's vocal performance, the trio successfully made a song that captured the spirit of the movie.

"I try to pick a place in the film where I can say, 'if we're gonna have anything, this is the moment we're gonna want to hear it,' " Silvestri says of his process. "For 'Believe,' we went to Santa in the town square of the North Pole. If we're gonna wanna go back and bring out the tune, it's gotta work here. That then becomes a place to get your inspiration from and you start hunting for notes that are gonna ring the bell, as they say."

When Groban was added to the process, he watched a few clips of the film "to get the vibe" and understand what tone the final song needed to hit. "An end title song is also generally hugely based on the main musical theme of the score, so it was important to work it into that," Groban says. "I loved what I saw and was excited to dive into the song."

Ballard says "Believe," which has the refrain, "When it seems the magic slipped away / We find it all again on Christmas Day," "captures the whole sense of wonder and mystery and the sense of what you can feel as a child somehow dissipates as you get older."

Silvestri echoes his collaborator's sentiment, honing in on what makes The Polar Express so particular. "There is a dark side to not believing and so Zemeckis was very clear that he didn't just want a bright little shiny ornament, he wanted some dimension to it," the composer says. "He wanted us to feel a bit of emotional range in the whole film, which I think he accomplished."

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Silvestri estimates that with Zemeckis' most recent movie, Here starring Tom Hanks, he has worked on 23 titles with the filmmaker, starting back in 1984 with Romancing the Stone.

Despite the fear of messing up what had the potential to cinch the whole movie together, Silvestri says their teamwork, the addition of Ballard and Groban and their trust made the song what it came to be. "Glen is not just a masterful songwriter and producer and artist, but he's a masterful storyteller. We met with Josh and he was up for it, and Warner Bros. was up for it. And so now we had all of this tremendous support around us," Silvestri says.

"Any opportunity I get to work with Alan Silvestri is, is a happy day for me," Ballard adds. "He's my favorite collaborator. I love him as a human being, as an artist and as a friend. I think we resonate because we're both storytellers so our relationship is a deep, entangled, beautiful thing. Anything that I can do with Alan Silvestri, I'm there and I'm gonna be early."

Groban, for his part, says he "learned so much from Glen and Alan" that he has taken into his projects that came after. "I’m in awe of their body of work and how versatile they are. Glen and Alan are all about feeling. They always write in ways that make me feel so good and as a vocalist it was such a pleasure to collaborate with them. I’m so proud we did it in a way that was endured year after year," he says.

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Most of the crew came together to watch the final film at the Stag Theater on George Lucas' Skywalker Ranch, an experience Silvestri likened to the relief of finishing a marathon. "You just are amazed at how much everyone worked to have that happen. That's a Christmas miracle right there," he says.

Groban then saw the film for the first time at the London premiere, which he says "was such a special way to experience it." The Grammy nominee quickly saw the potential for the movie and "Believe" to become modern classics. "With how special the story has been to me and to so many generations, I had a feeling it might be big," Groban says. "It made me feel all the good things when singing it and I just had a feeling it might hopefully be a yearly special song for people."

As the film hits its 20th anniversary and remains an annual watch for many, Ballard says, "I'm just so proud to be involved with something that is this unique and wonderful. And it continues to enchant people, and I think it will for a long time to come. I'm still on the train. I might be in the caboose, but I don't care."

The Polar Express is available to own on 4K UHD and Digital. Listen to “Believe” and Groban's new version of “Do You Hear What I Hear” with The War and Treaty wherever you stream your music.

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