Experience or formula?
Usually, however, there isn't much variation, because often times the central power desk will stipulate that a certain train ALWAYS has to run with at least 2 units because those units are then needed on another train. So, even if that particular day's train tonnage may be low, you still have to send it out with 3 units otherwise the power desk will have kittens because you've now totally screwed up the outbound train planning for the next couple of terminals. And, if you've got more tonnage than normal, there isn't usually an extra unit sitting around with nothing to do, so you reduce tonnage instead.
Thathought also occurred to me because I was the only passenger on a 20-seat commercial flight from San Francisco to Santa Barbara. I asked why they flew it rather than have me depart later or go on another airline. They needed the aircraft to depart from Santa Barbara.
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