It's called the "Big Boy," and it's trekking "The Great Race Across the Midwest."
Enthusiasts can come see the locomotive up close all weekend in West Chicago.
Union Pacific's Big Boy steam locomotive travels through Chicago, honors 150 years since First Transcontinental Railroad
It's just one of 25 steam engines built by Union Pacific in the 1940s. The "Big Boy" weighs in at 1.2 million pounds and is 133 feet long.
"This is a living exhibit of American history and machinery and it is just awesome to see," said Chris Goebel.
Steam locomotive rolling through Chicago
The last time a "Big Boy" locomotive was in operation was 60 years ago.
"I wish my mother was here to see it," said Regina Smith. "I am sure she remembered these from the '40s."
After departing from Butler, Wis., at 8 a.m., the train traveled through Wadsworth, Northbrook, Des Plaines, Elmhurst, and Wheaton and before ending up in West Chicago Friday afternoon, where it will be on display this weekend.
"Railroads, they're just this modern big network, but they are operating behind the scenes, kind of in the background of our lives and we don't really realize the vital role that railroad play," said Ed Dickens, locomotive engineer.
World's largest locomotive Big Boy coming to West Chicago
Park Ridge resident Bob Halston, 92, said he could feel the machine's power.
"It almost blew me off this chair," he said.
Halston made the event a family affair.
"My dad was always a rail fan," said St. Charles resident Mike Halston, Bob Halston's son. "He would chase trains, when we were kids he would bring us out. So this time, we brought him out."
Some people said they took time off from work to come get a glimpse of this big piece of history.
You can track the steam locomotive as it moves through the Midwest by clicking here.