Big Boy No. 4014 is the largest steam locomotive in the world. 

How many chuggas come before a choo-choo? We may never agree on that one

But, we do know for sure that a legend is chug chugging into town for a brief appearance this week. As of Monday afternoon, Union Pacific's historic Big Boy No. 4014 steam engine was in Southern California, and is on track to roll in to Tucson on Thursday. 

Here are 5 things to know about this massive machine. 

How big is Big Boy?

Real big. This machine’s a beast at 133 feet long, 17 feet tall and weighing 1.2 million pounds. 

Why it’s on tour:

A big birthday deserves a big celebration, and that’s why Union Pacific chose to spend several years restoring No. 4014 in honor of the 150th anniversary of the completion of the transcontinental railroad. Tucson is one of the stops on the Great Race Across the Southwest, the third and final leg of a cross-country trek to celebrate the milestone anniversary, according to a news release from Union Pacific.

Why it’s so special:

Big Boy No. 4014 is the largest steam locomotive in the WHOLE WORLD, and the only engine of its kind that’s still in operation. Union Pacific’s Big Boy trains, 25 in total, were delivered in 1941 and only eight are still in existence. The other seven Big Boys are on display across the country in seven cities including Dallas, Denver and St. Louis. No. 4014 was in service for 20 years and traveled 1,031,205 miles before retirement.

How to track it:

You can get real-ish time updates about the train’s location via its Twitter here or on Union Pacific’s website here.

Where to see it:

The train rolls in at 10 a.m. at the Historic Downtown Train Depot on Thurs. Oct. 17th, but will be on public display on Friday, Oct. 18th from 9 a.m. to 3 p.m. at the train yards at East Silverlake Road and Fairland Stravenue.