ROCK SPRINGS — The world’s last operating Big Boy locomotive rolled through Rock Springs on its way west as part of a coast-to-coast tour, drawing crowds eager to glimpse the 80-year-old engineering icon.

Union Pacific No. 4014, known as the “Big Boy,” is the only one of its kind still in operation. Twenty-five of the massive steam locomotives were built during World War II, and all of them were retired as diesel engines proved more fuel-efficient and easier to maintain. Union Pacific donated eight of them to museums across the country.

Mike Jaixen, senior manager of communications for Union Pacific, said the railroad reacquired No. 4014 around 2013, bringing it back from a museum in California, to the company’s steam shop in Cheyenne.

“They tore it back completely,” Jaixen said. “We inspected everything and rebuilt any parts that had shown wear or rust from sitting for 60 years.”

The locomotive was returned to operation in 2019.

Because the Big Boy runs on 80-year-old technology, maintaining it requires far more hands-on labor than a modern diesel locomotive. Crews stop roughly every 80 miles to lubricate bearings and perform other service along the tracks, a practice dating back to when steam engines were common workhorses in the 1940s and 50s.

“We baby it a lot more than they probably did in the old days, only because we’ve got one of these,” Jaixen said. “We want to make sure we bring it to the next stop, because there are people that can’t wait to see this.”

The tour will take the Big Boy to the West Coast, where it will stop at the Central Pacific’s Mile Post 0, before turning east. After returning to Cheyenne for additional maintenance, the locomotive will resume its eastward journey across the Union Pacific network into Chicago, then onto the Norfolk Southern network, with a planned arrival in Philadelphia for a Fourth of July celebration.

The Big Boy will depart Rock Springs for Green River 9 a.m. Wednesday.