The Union Pacific engine 844 was the last steam loco built in the USA. The engine has been preserved. I like this info-graphic about the loco; although it should be a little more Klutsis…
The machine isn’t the biggest loco; but it is still massive and was designed to haul 1000 tons at 100mph for hundreds of miles.
The power required to meet the spec needed a massive engine with a big boiler and a huge fire-box. Accordingly, you end up with extra wheels to support the boiler and fire-box and an automatic stoker to feed coal into the fire-box. No human engineer could keep pace with the machine.
Here is a picture of a model of the loco…I love the lettering on the tender. In general, and probably because of the larger sizes of the machines and the vastness of the landscape, the lettering on US trains is big and bold…
There are lots of great videos online too…
I’ve posted before about these monsters, here