@cullmann from what I understand, it is related to what you wrote in https://cullmann.io/posts/kde-qt-highdpi-scaling/ ? How is it only related to X11?

The old KWindowEffects API for this theoretically had explicit support for atomically highlighting multiple windows at once via its signature, but I'm not sure this needs preserving in pfactice.

Well, I do speak French, so I inquired about it directly to ressources.data.sncf.com. I'm not sure I'll get an answer, though. Meanwhile, I explored a bit their data API, there is a trove of interesting things.:

I am unsure if that's what you want, as I am not familiar with the format of the codes you expect: https://ressources.data.sncf.com/explore/dataset/referentiel-gares-voyageurs/table

I looked into this some time ago. I think https://transportr.app/ handles it in some cases, trough their own server, that uses https://www.navitia.io/ (a data aggregator, from what I can see).
I think they use their own open source server, which has the API keys.

I usually activate USB tethering on the phone side, and it "just works".