well, if you deliver 78 items per load and your mill consumes 400 per year, your frequency for the lines should be about 140 seconds. if you make it so that the frequency is 520 seconds (3 times less frequent), you should still be within 20 minutes limit to be able to deliver the materials, and you can haul 3 times more items per train (234 or so). to achieve that, use less trains, or use slower trains, or haul materials from further source (or some combination of those). I hope you get the idea..
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