It's longer than two buses, weighs more than a Boeing 747 fully loaded with passengers and can pull 16 Statues of Liberty over a mountain.

The Big Boy No. 4014 steam locomotive rolled out of a Union Pacific restoration shop in Cheyenne, Wyoming over the weekend for a big debut after five years of restoration.

It then headed toward Utah as part of a yearlong tour to commemorate the Transcontinental Railroad's 150th anniversary.

Big Boys hauled freight between Wyoming and Utah in the 1940s and 1950s. Of the 25 built by the American Locomotive Company in Schenectady, New York from 1941 to 1944, only eight remain and only No. 4014 will be operational.

Engineered for steep mountain grades, each Big Boy had not one but two huge engines beneath a 227-tonne boiler able to hold enough water to cover an area the size of a basketball court to the depth of a high-top shoe.

Union Pacific hasn't said how much the restoration cost, but Wrinn estimated at least $US4 million ($A5.7 million) based on similar restorations. The result will be one of just six to eight steam engines still operational on mainline US railroad tracks.

Retired Union Pacific employee Jim Ehernberger remembers the Big Boys well. He joined the railroad at age 16 in 1953.

"You could sure tell when a Big Boy left town. The ground kind of vibrated a little more than it did with the other types of locomotives. They were very, very powerful," Ehernberger said.

Union Pacific towed Big Boy No. 4014 to Cheyenne in 2014 after acquiring it from a museum in Pomona, California.

"They had to basically completely disassemble the locomotive down to just the frame and the shell," Trains Magazine editor Jim Wrinn said. "It was an immense undertaking."