On a train vs in a train

This question may seem a bit simple, but there is more to it than meets the eye. I frequently use Southeastern service in London (trains) which is equivalent for polish "PKP". While on a train there are multiple announcements, let me quote one of them: "Please do not leave unattended items of luggage IN the train or on the station." As far as I know, the phrase "in the train" is commonly considered incorrect. I wouldn't have asked this question if it wasn't for the fact that I've heard that announcement literally a thousand times. Could anyone please shed some light on this problem. PS. I also asked my native speaker friends and they say that "in a train" sounds awkward, which is fine with me as I also consider it as sloppy English. This spoken phrase is being played nearly all the times while travelling by train, so it's really strange to me that no-one had changed it, unless there is some kind of an exception that makes this phrase acceptable or correct. You can also read this thread: https://www.etutor.pl/pytania-jezykowe/pytanie/10237-on_the_train.html Thank you.