The 'Big Boy' and 'Challenger' steam locomotives were the most
efficient, and also the largest (in the US anyway).
From Wikipedia -
https://en.wikipedia.org/wiki/Union_Pacific_Big_Boy
"The Union Pacific Big Boy is a type of simple articulated
4-8-8-4 steam locomotive manufactured by the American
Locomotive Company (ALCO) between 1941 and 1944 and
operated by the Union Pacific Railroad in revenue
service until 1962.
The 25 Big Boy locomotives were built to haul freight over
the Wasatch Range between Ogden, Utah, and Green River,
Wyoming. In the late 1940s, they were reassigned to
Cheyenne, Wyoming, where they hauled freight over
Sherman Hill to Laramie, Wyoming. They were the only
locomotives to use a 4-8-8-4 wheel arrangement: four-
wheel leading truck for stability entering curves,
two sets of eight driving wheels and a four-wheel
trailing truck to support the large firebox.
Today, eight Big Boys survive, with most on static display
at museums across the USA. One of them, No. 4014, was
re-acquired by Union Pacific, and between 2014 and 2019
it was rebuilt to operating condition for the 150th
anniversary of the first transcontinental railroad. It
thus regained the title as the largest and most powerful
operating steam locomotive in the world."
