123Movies. Meet the crew of the Union Pacific Challenger No. 3985, the largest and most powerful steam engine in the world. This colorful documentary is a behind-the-scenes look at the conductors, engineers and mechanics who keep this fickle train running, an engaging history of steam power, and a scenic tour the Great Plains -- from Cheyenne, Wyo., through Denver and across Nebraska to the Omaha headquarters of the Union Pacific.