Technology Engineering World's largest steam locomotive heads out on tour Union Pacific's Big Boy No. 4014 will travel coast-to-coast in commemoration of the semiquincentennial. Breakthroughs, discoveries, and DIY tips sent six days a week. The world's largest operating steam locomotive is hitting the road--or tracks . Union Pacific's Big Boy No. 4014 is heading out on its first coast-to-coast steam tour to celebrate the United States' 250th anniversary . The first leg begins on March 29, when Big Boy and other historical passenger cars from Union Pacific's Heritage Fleet will travel from the locomotive's home base in Cheyenne, Wyoming, west towards California.