Trans-Siberian Orchestra, the rock-opera brainchild of Paul O’Neill, has become an unlikely holiday juggernaut since its mid-’90s formation, with album sales of more than 12 million and ticket sales in excess of 14 million.

But when O’Neill suddenly died this past April from a reaction to the combination of prescription medications used to treat several chronic illnesses, it was fair to wonder — would TSO continue without its primary composer and lyricist?

That, apparently, was never in doubt. TSO’s 2017 Winter Tour, in which the group will perform its hit production, “The Ghosts of Christmas Eve,” kicks off this week nationwide, with two shows at Salt Lake City’s Vivint Smart Home Arena on Tuesday, Nov. 21, at 4 and 8 p.m.

Trans-Siberian Orchestra

When • Tuesday, Nov. 21; shows at 4 and 8 p.m.

Where • Vivint Smart Home Arena, 301 W. South Temple, Salt Lake City

Tickets • $43.50-$73.50; Ticketmaster

TSO’s musical director, Al Pitrelli, said on a conference call that while O’Neill’s presence is missed, it was his family’s wish that the show go on.

“Paul and his family, Paul and his wife — this was their child that they gave birth to years and years and years ago,” Pitrelli said. “It’s so nice to know that the family is going to carry on the legacy. TSO is one of their children. I’m just glad to be part of it. Whatever they want to do, we’re good with.”

That means continuing to offer the fundamentally bizarre yet somehow totally workable combination of holiday-themed material performed by a string section and an arena-rock outfit, all accompanied by the histrionic bombast of flames, fireworks and laser lights.

TSO began touring for live performances in 1999, and audiences continue to come back for the extravaganza.

“We’ve become such a tradition. We’ve become to people what ‘It’s a Wonderful Life’ or ‘Miracle on 34th Street’ was to me when I was kid,” Pitrelli noted. “This is something that people have latched onto and made part of their holidays.”

He doesn’t expect that to change now.

For one thing, Pitrelli said, the song lineup in “The Ghosts of Christmas Eve” is “almost the greatest hits collection anyway, so everybody will hear their favorites.”

Further, he added, even when TSO resurrects popular programs, the musicians make it a point to update the proceedings.

“The production is always different. The look of the stage will be different, the lighting, the pyro, the lasers, the moving trusses, the video content,” Pitrelli said. “We’ll always try to upgrade that from year to year because we never really want to repeat ourselves.”

(Photo courtesy Bob Carey) Trans-Siberian Orchestra music director Al Pitrelli said he still can’t believe how big TSO has become: “We started making some records back in ’95 and ’96, and ’99 we started touring — we had one box truck and a couple lights and a fog machine, a vision and a dream. Little by little, circuitously, we got to the point where we are right now. It wasn’t overnight.”

He couldn’t help but marvel at TSO’s growth from its humble beginnings.

Seeing Trans-Siberian Orchestra grow from O’Neill’s initial concept to the full-blown spectacle it’s become makes him simultaneously awed and proud.

“Paul and his family, this is their child. I’m kind of like the weird uncle. I don’t know. I’ve watched it from infancy to what’s going on 22, 23 years now. Figuratively speaking, or metaphorically, I’ve watched this baby graduate college with honors. I’ve watched it go out into the world, I’ve watched it do things that nobody could ever thought it would do,” Pitrelli said. “It’s exceeded everybody’s expectations. I mean, we started making some records back in ’95 and ’96, and’’99 we started touring — we had one box truck and a couple lights and a fog machine, a vision and a dream. Little by little, circuitously, we got to the point where we are right now. It wasn’t overnight.”

TSO will continue to evolve.

According to Pitrelli, O’Neill and his writing partner, Jon Oliva, had several more TSO projects already in the works, and estimated that several albums were already “halfway done, 60 percent done,” thanks to recording done in the downtime between tours.

“Fortunately, Paul was a workaholic and he has plans for tours and production ideas a couple of years ahead of where we are right now,” he said.

So, TSO isn’t going anywhere. Or rather, it’ll still be going everywhere.

Pitrelli has played no small part in keeping TSO’s finely tuned machine running, though he downplayed his significance to the effort, saying most of the musicians in the ensemble can be counted on to take care of themselves, and those who can’t don’t last very long.

“I’m in the very fortunate position of keeping my eye on the integrity of the music and the stories and the delivery live,” he said. “Being a musical director is being like the conductor of a really, really good orchestra. You start these guys and stop them, and when you surround them with such incredible talent, you really don’t have to do much more than that.”

The guitarist couldn’t say yet what the mood of him and his fellow performers might be when they take the stage for the first time without their former boss to oversee things.

So for now, he’s just focused on not screwing up O’Neill’s legacy.

“The only thing I could compare it to is going to my first Thanksgiving dinner after my father died. Something’s missing, but the family carries on, and we’ll celebrate my dad’s life, we’ll celebrate Paul’s life. … There will be an empty hole in everybody’s heart, but life will continue to go on,” Pitrelli said. “He’d say he wants this to live forever. He wants our children and our children’s children to be aware of what we did. People used to compare it to Pink Floyd and [Emerson, Lake & Palmer], and things like that, and Paul chose to try to compare it to the works of maybe Mozart and Beethoven. He wants people to remember this 200 and 300 years down the road, not just 30 or 40 years.”