Spectators take pictures of Big Boy No. 4014 in Covina, Calif., earlier this year. The huge steam engine, which has been part of a museum display in California, will be restored by the Union Pacific Railroad and will pull tourist trains.

CHEYENNE, Wyo. — In its prime, a massive steam locomotive known as Big Boy No. 4014 was a moving eruption of smoke and vapor, a 6,300-horsepower brute dragging heavy freight trains over the mountains of Wyoming and Utah.

It’s been silent for half a century, pushed aside by more efficient diesels, but now it’s coming back to life. The Union Pacific Railroad is embarking on a yearlong restoration project that will put No. 4014 back to work pulling special excursion trains.

“It’s sort of like going and finding the Titanic or something that’s just very elusive, nothing that we ever thought would happen,” said Jim Wrinn, editor of Trains, a magazine that covers the railroad industry.

“Something that’s so large and powerful and magnificent, we didn’t think any of them would ever come back,” he said.

The American Locomotive Co. in Schenectady, N.Y., built 25 of the monsters to Union Pacific’s specifications between 1941 and 1944, and they became legendary. They were the largest steam locomotives ever to work the rugged terrain of the American West, and by most standards the largest anywhere in the world, said Gordon McCulloh, a meticulous historian of Union Pacific steam power.

Even the name is legendary. An unknown worker scrawled “Big Boy” on the front of one of the engines when it was under construction.

“It came out one day, and it had ‘Big Boy’ in chalk on it. And from that day forward, it was Big Boy,” said Ed Dickens Jr., Union Pacific’s senior manager of heritage operations, who will oversee the restoration at the railroad’s steam shop in Cheyenne.

Dickens and his crew recreated the chalk inscription on No. 4014 when they began to move it.

Big Boys are 132 feet long, including the tender, which carried coal and water. They weigh 1.2 million pounds with a full load of fuel. They are essentially two engines under one boiler, with two sets of eight drive wheels, each set powered by two enormous cylinders nearly 2 feet across. Big Boys are so big that the front set of drive wheels has to pivot separately from the back set to get around curves.

And they aren’t just big, McCulloh said. They were engineered to reach 80 mph, even though the railroad never intended to run them that fast. The point was to fine-tune the locomotives so they stayed in balance at any speed and didn’t beat themselves up with their own powerful forces.

“You get all that machinery to live in harmony,” McCulloh said.

Their enormous bulk also hid some slick engineering, including a suspension system that kept the drive wheels pressed against the rails when the locomotive straddled hills or valleys.

Seventeen Big Boys were scrapped when they were pulled from service, but eight survived and are on display around the country. Union Pacific chose 4014 for restoration because it spent more than 50 years in the friendly climate of Southern California, at the RailGiants Train Museum.

The museum took good care of the locomotive, Dickens said, and many of the original components are usable.

RailGiants agreed to give the Big Boy back to the Union Pacific in exchange for some other equipment to display.

No. 4014 has been moved from the museum to a Union Pacific shop in Colton, Calif., where a crew is preparing to tow it across Nevada, Utah and Wyoming to Cheyenne, arriving May 8. The AP maintains two other working steam locomotives there, and Dickens and his crew of seven can manufacture almost anything they need.

“This equipment, nobody’s going to come in and fix it when it breaks down,” he said. “We’re it.”

Restoration is expected to take three to five years. The railroad would like to have the Big Boy operating by 2019 for the 150th anniversary of the driving of the Golden Spike in Utah, which linked the Union Pacific with the Central Pacific and completed the first transcontinental railroad.