As any normal person. You open the door. You enter the area where you can sit down, the nice and heated area. Decent seating and not to much noise. You just left the area that is not heated, that has less sound insulation and is in general just a corridor. So as anyone would do, you look behind you, look if there's someone behind you and you close the door. This will keep the cold away and it's quieter in the carriage that way as well.

But nope. People will never close the door behind them. Only very few people do that. I don't get it, what's so hard about closing the door? Do you want to have loud buzzing sounds from the wheels and rails and hear every little thing that happens outside the train? Do you want the heat to leave the carriage? I don't get it

And now I'm going to sound like an old grandpa sitting in first class and say: it's mostly students that "forget" to close the door. What's wrong with the youth, aren't they educated at home and taught to always close a door behind them. That what door are for. Their only task in existence is to open and close, and so separating the 2 areas, as intended by design.

I just don't get it, why don't people close the door on te train? Yes this matter. It's freezing cold outside and I don't want to freeze to death while going to home or work. (or get frostbite, which is a real possibility since I suffer from wintertoes and fingers, painful condition)

Disclaimer: this only applies for trains with manual doors property of the NMBS/SNCB.