Op, is it possible to make bulk commodities ( food, metal... ) in huge quantities and a little more pricier and precious commodities ( diamants, gold... ) in small quantities ?
2000 tons of food is realistic but 2000 tons of diamants is strange and 2000 tons of military secrets is uncommon.
So big cargo's ( trains ) can remplish the cargoload with 1 kind of cheap ( ? ) commodities or several different precious commodities, letting ships with small cargoloads going for high precious or illegal commodities.
it's a very old idea but maybe it's doable with all the technical developments you make these last years!