With plumes of smoke billowing into the sky, the world's largest steam locomotive left Wisconsin on Friday morning, bound for an afternoon stop in West Chicago.Back on the rails for the first time in six decades, the only operational Big Boy steam engine in the world is on a journey across the Midwest to mark the 150th anniversary of the completion of the Transcontinental Railroad in 1869.Just 25 of the Big Boy engines were built, in the early 1940s, primarily for hauling freight through the Wasatch Mountains in northeast Utah, according to Trains magazine.After logging more than 1 million miles on Western railroads, engine No. 4014 was retired in 1961 and given to a chapter of the Railway and Locomotive Historical Society in Southern California.Union Pacific engineers have been doing restoration work on 4014 for the past two years in Cheyenne, Wyoming.The steam engine stopped Tuesday in Altoona, Wisconsin, about 90 miles east of Minneapolis. Thousands gathered to take in the sight.

"It's kind of an emotional experience for us, because people are so excited to see the locomotive," Ed Dickens, who leads Union Pacific's heritage operations, told CNN affiliate WEAU. He said the joyful crowds made the 2½ years of work on the locomotive worthwhile.

The 600-ton locomotive left Butler, Wisconsin, at 8 a.m. Friday en route to Chicago, Trains magazine says.

Union Pacific's schedule says the Big Boy will sit on display in West Chicago for events through the weekend. It'll then head west to Rochelle, Illinois, on Tuesday morning and will traverse Iowa, with stops that include Cedar Rapids and Des Moines.

By August 2, the train will be in Omaha, Nebraska, for the weekend, and it'll spend the first half of the week making stops in Nebraska on the way back to its original perch at the Union Pacific Steam Shop in Cheyenne on the morning of August 8.

Union Pacific is posting updates on the train's location on its Twitter feed.