The Union Pacific Railroad’s enormous showpiece engine is chugging back through the Hawkeye State. U-P spokesman John Bromley says the Challenger is the world’s largest operating steam locomotive. The locomotive is 122-feet long and it weighs more than a million pounds. It can run at about 70-miles an hour. It was built during wartime — in 1943 — for “fast freight” service. Bromley says the big locomotive is used by the U-P as its rolling ambassador. It’s among two steam locomotives the Omaha-based U-P maintains for public relations, excursions and special events. It’s cutting through Iowa on its way back to its headquarters in Cheyenne, Wyoming. Bromley says the 61-year-old locomotive has a permanent crew of eight. The big machine saw service for about 16 years, starting in 1943.Challenger was retired in 1959 when steam service was phased out. It was restored to running condition in 1981 by volunteer U-P employees for its special duties. The big locomotive will be chugging across Iowa on Friday — headed from Trenton, Missouri, to Boone. It’ll be on display in Boone overnight and departs Saturday for Fremont, Nebraska. For more information, point your computer mouse to “”.