The only thing I can think of is that there must be different types of train cars with different widths above ground, even though the wheel base is the same. So depending upon whether the car is a narrow one or a wide one, they use the appropriate track. A narrow car uses the one closest to the platform to minimize the gap climbing aboard. And a wide car uses the far track so that it doesn't smash into the platform. Woe be to the conductor who who puts the wide car on the narrow rail... That's my guess, but I know nothing about trains.
