I'm not sure, how this can be handled. To get the right amount of items from a to b with belts, you're not only able to configure the poduction rate but also the belt speed (by choosing the according MK). With trains it's a bit more difficult.
It depends on the length of the train route, the amount of items produced, the amount of items needed at the receiving station and the number of freight cars. The train speed is always the same.
To take the full advantage of the train speed (and amount it can load), it's in fact about ore only. Except you have a super mega max factory which produces 2500 heavy modular frames a minute (or you move something from one edge of the map to the other).
So in my case, even with only one freight car, my train begins to ride back to the loading station with a half full freight car, cause the unloading station is full. To avoid this, we need more configuration features into the timetable of the train. This could be a waiting time at a station for instance. Or the train waits automatically at the unloading station, until its freight car is empty. Another way could be, that the train automatically waits at the loading station, until the unloading station has enough slots.
I know, all that only would work, if you move one kind of items only with one train. Maybe someone has a better idea.