The biggest fanticy loco I have know is a double Big Boy Garrett. That's 4-8-4 + 4-8-4 + 4-8-4 + 4-8-4. All engines were on pivots. Alco had patent rights to build Garretts in the US but none were ever built. I think Alco also considered a quad Challenger but that went no where also. My opinion is that the Garrett type engine is the only way to get huge engines around curves of reasonal radius. Then there is also the size of the boiler and its heating area to feed all those cylinders. 1,000,000 Lbs +?
Cary