I’ve also learnt the word I would have needed to type into DuckDuckGo to learn about the economics involved:
#MarginalCost. It’s jargon related to economies of scale, that when translated to transportation means loosely the following: the increase in total cost [of a mode of transportation] by increasing the [journeys taken] by one. I could rephrase the definitions in terms of marginal cost, but I’m afraid to get it wrong, so I’m not going to.