They were called Big Boys, and they were indeed huge in the public’s mind. The “Big Boy”, Union Pacific’s Class 4000 steam locomotives, may not have been the biggest in every dimension or measurement (see that discussion here), but they were biggest where it counted, in public relations for Union Pacific, in the hearts of the public, and in longevity – outlasting all competitors for the “biggest” crown.
In fact, there’s a Big Boy running today. On this page you’ll find history of the Big Boy locomotives. For the story on how one of these leviathans is running in the 21st century, check out The Story of Big Boy 4014.
By the way, steam locomotives are properly referred to as “she” or “her”. It seems odd calling a machine, let along one called Big Boy, a female pronoun, and railfans have had some pretty heated discussions about that subject over the years! We attempt some enlightenment with “Hey, Big Boy is a HE, why did you call Big Boy a SHE?“. Enjoy all our Big Boy articles!