Rail fans thrilled to see Union Pacific Big Boy locomotive up close in West Chicago, Illinois

A massive steam locomotive that first went into service 85 years ago has been traveling coast-to-coast to celebrate America's 250th birthday, and this week, it made a stop in Chicago's western suburbs.

After rolling into the area on Tuesday, the Union Pacific's Big Boy No. 4014 made a stop in West Chicago at the Union Pacific Training Center in West Chicago.

The Wallen brothers — 8-year-old Jackson and 5-year-old Nathan — had an endless supply of steam as they blew toy train whistles while waiting to see the Big Boy.

They waited in line with their mom and grandma, and thousands of people, to see the hulking Big Boy — which stops everyone in their tracks.

The Big Boy is the world's largest operating steam locomotive. Amateur photographer Joanathan Chapman couldn't even get a shot of it in one frame.

"It's a fairly wide-angle lens," Chapman said as he pointed his vintage camera. "It's still not wide enough from where I'm at."

Meanwhile, Jackson Wallen saw sone chalk writing on the locomotive that might be historically significant.

"In late 1941, an anonymous worker wrote those words, 'Big Boy,' and the front of the locomotive, and the name Big Boy stuck," said Ed Dickens, senior manager of heritage operations for Union Pacific.

Dickens helped restore the 85-year-old locomotive.

"Took us two and a half years. We took it completely apart," he said. "7,000 horsepower. One of 25 built."

Eight Big Boy locomotives are left, but the other seven are permanently stationary now. No. 4014 is the only one that still runs on rails.

"It looks just really cool," said Jackson.

Currently, the Big Boy is on a journey across the country, from California to the Eastern Seaboard, all for the 250th anniversary of the founding of the U.S.A.

"I love this. I love the people," said Dickens. "I mean, look at in the crowds there, there's so many people with model trains, with books, with pictures, sharing stories, memories of their families, experiences, their connection to the railroad."

Young Jackson Wallen was wonderstruck.

"I didn't think there was going to be a bell on it. I didn't think there was going to be like that — there's like an eagle on it," said Jackson. "I didn't think there was going to be that."

The unexpected adds to the excitement that reached its peak with the sounding of the Big Boy's steam whistle — which uses 300 psi of live steam heated to 700 degrees, and can be heard 4 to 5 miles away.

Viewing of the Big Boy in West Chicago ended at 3 p.m. The locomotive gets going again on the rails on Thursday.