First of all, it is not possible to say that that there is a “positive correlation” between people’s age and where they sit in a train car. The reason for this is that there is not positive and negative in train car seating. A positive correlation exists when an increase in one variable goes along with an increase in another variable. Age can increase. Where you sit in a train car cannot. So, to be accurate, you will need to say something like “there is a positive correlation between one’s age and how far forward in a train car one sits.
Outside of that, I think you would need to clearly define what kind of train car you are talking about. For example, a train car that has one door at the “front” (closer to the direction it is travelling) might yield different seating arrangements than a car that has a door at the back. Both of these might be different than train cars that have doors that open in the middle.