Apparently the strongest European fast train steam loco was from Austria and produced under license in Rumania.

https://de.wikipedia.org/wiki/BB%C3%96_214

I think there is always going to be some controversy or debate on many aspects of the biggest, longest most powerful etc. Every country has its favourite locos and wants to promote them all.

Same as the record speeds recorded, some say other trains did 150 mph but was not recorded as a record attempt etc. All the steam era trains are magnificent regardless of position on the worlds best whatever list

The Big Boy was one of the most successful large steamers, but not the largest by several measures. The Yellowstones and the Y-Class were massive in both size and power but they were drag locos that pulled long ore/coal trains at very low speeds. Several steam locos made more drawbar HP than the Big Boy and a few were longer, although I would need to do some research to enumerate them.

What is so impressive about the Big Boy is that even with all of that power they were able to safely run up to 130 kph and were capable of occasional operation at even higher speeds. They were normally used to pull long and heavy trains over mountainous terrain at high speeds without a helper. The technology that allowed a large articulated loco to run so fast with good tracking was developed by ALCO and UP with their very successful Challenger. UP then went on to expand on the Challenger concept to create the Big Boy and a shop worker with some chalk made history with his flippant name for this beast.

I've often wondered though, were US steam locomotives larger in general than most locomotives of the same type used in mainland Europe? Assuming that most of the models I have are at least close to being accurate in scale, the few remaining US steam locos I have are noticeably larger than similar German steam locos I own. For example, the brass Southern Railway (US) Ps-4 I own, which is based on the USRA Heavy Pacific design, is larger in every dimension than my BR-01. My guess is that the tunnels initially dug in Europe were never enlarged over time, and locomotives had to be built to fit, while tunnels are much less common in the US, and those that were built were either dug larger initially or enlarged later.

Maybe one day I'll start another thread, and take some photos...

For me is the biggest loco at this list #10. Exceptional a new class of locos: the electrical steamer!

The Norfolk & Western wh(Y)-class!

https://commons.wikimedi...(1918)_(14778733323).jpg

https://largest.org/technology/steam-locomotives/

Wonderful.