I think that there are plenty of TRAs about who are engaging with single sex services in order to see what happens, kicking up a fuss if they are not accommodated how they want, and in the current climate companies fold very quickly.
This is the topshop thing >> a man who is not a TW but "gender fluid" wanted to use the girls fitting rooms. They said no. He kicked off. They said yes, anyone can use whichever they choose, teenage girls in underwear and flimsy gappy curtains and all.
In this rail thing it's one protected characteristic against another. One has to "win". On top of this it's men v women, and men win, as this is how society is set up. I have never seen an example of a transman doing this sort of thing - has anyone else?
So - TW / gender fluid feels like a woman today person rocks up for their women's sleeper berth. They are in with a 15yo girl - because yes why not? I'm sure lone teens use these services for various reasons.
Actually in this case, the teen girl would probably be polite and uncomfortable and not say anything as that's the way we're socialised, and it is very hard to speak up when you're young.
OK so scrub that one, it's an older women who goes and complains to staff. What do they do? Sleeper trains are often full. They need to choose. Does someone need to get off? Who? My guess it would be the complainant ie the woman.
That's how I'd see it playing out IRL.