Smr Simple!

When I try to fix the route of a train, some stations sometimes can't be chosen. ie, some appear white on the map (they can be chosen), some grey (they cannot).

Asssume that any of the places to choose do have a station, a track directly connecting one to the other, and optionally a second track in the area of the station, if necessary.

What other reasons may block these stations from being selected?

This feature is reliable. It means your rail connections are not correct (redo), or you skipped one station you need to include in the routing (add it).