I rode in the cab of an English 2-8-0 when I was a kid. This was in the 1950s. The ride was extremely rough & scary. I don't think we got over 10 mph. I wouldn't have wanted to go any faster in that thing on that track.
I took the CZ from Chicago to Emeryville, CA two years ago (2022). The BNSF track between Chicago and Denver was very rough. I was nearly thrown from bed several times. I thought something was wrong with my Amtrak sleeper. The ride on the UP between Denver and Emeryville was silky smooth. There was nothing wrong with the sleeper.
My wife and I also experienced a rough ride on the Empire Builder between Chicago and Glacier Park in 2023. BNSF track, in my experience, is not as smooth as UP's track.
A railroad steam engine has multiple closely spaced fire tubes running the length of the boiler. (A fire tube carries the hot gases from the fire through boiler.) This will prevent most, but not all, of the free water effect (sloshing) of water back and forth. NH Joe