Ideally announcements should be bilingual, but if there’s only one language, it’s better to inconvenience any number of foreigners than even a single native who doesn’t speak English.
The point of an official language is that the government makes it as convenient as possible living your life knowing only that language. It wouldn't be that unreasonable IMO to switch official language to English, but seeing as NL is a democracy, that ought to be a democratic decision. A referendum of some kind would probably be best. It's not a small decision to be taken lightly.
But it's still nice to offer information in other languages in certain situations. Infrastructure used heavily by tourists and recent immigrants who don't speak the language yet should be made as usable as possible to people who don't know Dutch. Making important announcements in both English and Dutch on trains is an example of that.
