Comment by rostigerpudel

You get repeated hints like 20 times outside and inside the train. Announcements (also in english, recorded, by an english native speaker, repeatedly), the train displays explain it and when you get to the station where the train splits, every display in that train shows you whether you're in the right carriage and you get an extra announcement exclusive to the carriages that go somewhere else that you should change now if you want to go to the airport.

This really is the original poster's problem.