Child in Chapter 875 (Might contain spoilers))

[k11chi]

Also Sera said that the hat that she wore was a lot like her older brother's, but in every flashback that we've seen Akai(New York, Black organization, and current) he always wore a toboggan(knit cap, beanie).

That's not quite right. Sera said her hat has the same sort of feel as the ones her brother wore, but the wording is more like the fit or feeling of wearing it than physical appearance. The translators had a discussion over that line when the chapter came out that you might be able to find in the thread archive.

The next line was ''But, that's impossible.'' So, maybe she had a feeling Akai actually wore the hat (Which he did)?

The feeling Masumi has is that the hat feels like it was worn by her brother, but she follows up with "but that's impossible" because she thinks Akai is dead so he couldn't be him wearing her hat.

Honestly the way Masumi is reacting is confusing because she was confronted with someone who appeared like her brother on the train -- Scar Akai. I am guessing she has concluded that Scar Akai wasn't her brother because he attacked her - something the real Shuuichi would not do, and especially not with a tazer. Naturally that should lead to followup questions about why someone would attack her while in disguise as her brother, but Masumi doesn't seem to be investigating anyone about that which is odd. But then again the whole train case feels a bit sloppy logically, and followup speed by characters has been very inconsistent in the Bourbon and Akai family arcs, especially when Masumi is involved.

The most annoying logical flaw in the mystery train case was that the trick relied on the rooms openening outward in a narrow corridor.
doors don't open outward.
even if they did, why would that happen in a narrow train?

Well my outdoor does open outwards but the doors were shown multiple times (5+?) to open outwards so does it really matter if you actually pay attention?