It's because those trains want to have fun with you as you drive close to them!
No, It's because they want you to get out of the way. If you try honking at them, they'll that they have to get out of the way. And they have missions, just like you. and also another tip - the traffic will increase in a world the more you play on the world. the traffic level goes down to 0 when you are not in the world.