TL;DR: I have no idea!

(Extremely) amateur history time! This Train facts post has been going around, claiming that in 1825, the fireman aboard the Best Friend caused an explosion by
sitting on a relief valve, because the sound annoyed him.

But my guess is still about 50/50 on whether a butt was involved.

One site says the fireman on the Best Friend was actually a slave, which is an interesting bit of history in and of itself, and wiki says it was common practice to tie down valves for various reasons, like to add some extra pressure to go up a hill, but nowhere seems to mention any thicc rears being involved in this incident.

Many even specifically state that it was “tied down”.

In one museum replica there's something that almost kind of looks like a weight on a lever(Which seems to be how safety valves were done at the time) within reach of the fireman, but it is overhead. A more modern replica doesn't have it, some other illustrations don't.

Moving the thing out of reach was apparently a very early change right after the explosion, so I suppose the replicas might not be perfectly accurate, or might even be intentionally inaccurate for safety. There could well have been a valve in butt range.

There doesn't seem to be anywhere in the fireman's general area to put a valve that could be sat on though, except for obviously bad places. One presumably would very much want the thing overhead and close to the boiler. But, bad designs aren't exactly uncommon even now.

But on the other hand, we have this report(very dated language ahead!!!), saying that it was “Held down”, rather than being tied down.

“I have just returned from examining the situation of the Locomotive Engine “Best Friend,” since the accident of this morning, and have come to the conclusion that the bursting of the Boiler originated from an over-pressure of Steam, and believe it to have occurred from the Safety Valve being held down by one of the Negroes attached to the arranging of the Car, (while the Engineer was attending to the arranging of the Lumber Car) and thereby not permitting the necessary escape of Steam”

And, a page from mysticstamp.com claiming:

“Another account claims the fireman set a piece of lumber on the valve and then sat on it.”

And, a book called “Rails Across Dixie: A History of Passenger Trains in the American South” claims “one source” says he sat on it, with a footnote reference leading to page 77 of “The Age of Steam: A Classic Album of American Railroading”.

Which basically says the same thing, “The first negro ashcat on record sat on the safety valve of the South's first successful locomotive”, but there doesn't appear to be a reference.

Some of the books one finds while researching the topic also seem to reference the idea that railroads were sized to match roman roads, which Snopes makes a pretty convincing argument stating that is only partly true, making the whole thing seem less credible, like as if there's plenty we still don't know.

It's amazing how even in the internet age, questions like this can't be answered in just a five minute Google search.

It's even more amazing how actual history books and museum websites don't always seem to have a clear “One final answer for all”, even when they sound really confident.

A half hour later, I still have no idea if a posterior was involved. But it is really interesting to see an easy to understand case of how some of the details get hard to find over time, sometimes with random stuff crowding out the real stories.

If he did sit on the valve, is posterior thiccness likely had very little to do with it.

I imagine he would have probably been sitting on a small weight that doesn't need much to hold down. Anyone directly occluding an open steam vent with their bottom might find themselves in need of the(Probably quite terrible, as this was 1825) services of a doctor, or failing that the undertaker.

And, as this was a hard job(Like just about any job back then!), I imagine one would likely be reasonably thin.

What do you think happened?