Hundreds headed downtown Thursday morning to see one of the world's largest functioning steam locomotives power its way through Tucson, as part of a year-long trip to celebrate the 150th anniversary of the completion of the transcontinental railroad.

The locomotive — known as Big Boy No. 4014 — is one of eight of its kind still in existence, and only No. 4014 is operational. Twenty-five of the steam engines, called Big Boys for their size, were built for Union Pacific in 1941 and were designed to handle to steep mountain terrain between Cheyenne, Wy., and Ogden, Utah.

The 4-8-8-4 engines, with two sets of eight 68-inch driving wheels, are so long that their frames were articulated — essentially hinged in the middle — so that they could make it around the curves in the railroad. One of the 85-foot-long 772,250-lb. Big Boys, along with its 436,000-lb. fully loaded tender, weighs more than a Boeing 747. The engine and tender (which carries water and fuel oil; formerly the "coal car") together are 133 feet long.

Owned by Union Pacific, No. 4014 underwent a two-and a-half-year restoration, and the big machine is now on tour through the U.S, rumbling through Tucson after leaving Los Angeles, and will head toward El Paso, Texas.

A GPS map, available here, allows people to track the train's journey across the United States.

After parking for a brief reviewing at the Tucson Historic Train Depot, the train headed to the Union Pacific rail yard, and will be on display Friday morning from 9 a.m. to 3 p.m. at East Silverlake Road and East Fairland Stravenue. The Big Boy is scheduled to steam eastward out of town on Saturday morning, passing through Benson around 9 a.m.

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