I've just been assuming it means the train is heavier, as if full of passengers/freight. (I do have a vague memory of something like this being mentioned in a video stream very early on, but I could be getting it confused with Matt saying something about the physics of the trains responding realistically to have air conditioning turned on or off.)
Let's say you are on SPG and you place a coal train, it determines if the cars are full or empty. For freight cars that you can't see inside, it just makes them heavier to simulate they are loaded. Idk about passenger trains, I haven't tried it with those yet.